fix(mcp): an abandoned tool call tells the server to stop

`try_call_tool` wrapped `call_tool` in a bare `tokio::time::timeout`.
On a timeout or a cancelled turn the future dropped and the server kept
working: no `notifications/cancelled` ever left the client.

`call_tool_cancel_aware` sends the request through
`send_cancellable_request` and holds a guard armed with the request id.
A settled call disarms it. A dropped future (cancelled turn) spawns the
cancel from the guard's drop. A timeout sends the cancel inline, before
returning: the HTTP timeout arm resets the transport next, and dropping
the last service handle killed a spawned cancel before it left.

The inline send waits 500 ms at most. rmcp 2.2.0 awaits its own
timeout-cancel without a bound, and its streamable-HTTP worker awaits
each POST in turn, so against a JSON-mode server the cancel queues
behind the hung call: with rmcp's timeout path the call returned after
3.0 s against a 1 s budget. That server class cannot receive the cancel
until rmcp 3.x; the budget elapses, a warn is logged, and AGENTS.md
records the limitation. stdio and SSE-mode servers receive it.

Tests: dropped call, timed-out call followed by a service drop, settled
call; the HTTP outer-timeout test now asserts the return time.

Ported from upstream (grok-build mcp/servers.rs call_tool_cancel_aware,
1.0.33); upstream leans on rmcp 3.2's timeout path.

crates/codegen/kigi-mcp/src/servers/cancel_aware.rs

@@ -0,0 +1,224 @@
+//! `tools/call` that tells the server when the host stops waiting.
+
+use rmcp::model::{
+    CallToolRequest, CallToolRequestParams, CallToolResult, CancelledNotificationParam,
+    ClientRequest, RequestId, ServerResult,
+};
+use rmcp::service::{Peer, PeerRequestOptions};
+use rmcp::{RoleClient, ServiceError};
+
+use super::McpService;
+
+/// rmcp 2.2.0's HTTP worker queues the cancel behind a hung POST, so the wait is bounded.
+const CANCEL_DELIVERY_BUDGET: std::time::Duration = std::time::Duration::from_millis(500);
+
+/// A timeout cancels inline, before the caller can reset the transport; a drop cancels from a task.
+pub(super) async fn call_tool_cancel_aware(
+    service: &McpService,
+    params: CallToolRequestParams,
+    timeout: std::time::Duration,
+) -> Result<CallToolResult, ServiceError> {
+    let handle = service
+        .peer()
+        .send_cancellable_request(
+            ClientRequest::CallToolRequest(CallToolRequest::new(params)),
+            PeerRequestOptions::no_options(),
+        )
+        .await?;
+    let mut guard = CancelOnDrop {
+        peer: service.peer().clone(),
+        request_id: Some(handle.id.clone()),
+    };
+    let Ok(response) = tokio::time::timeout(timeout, handle.await_response()).await else {
+        guard.cancel_within_budget().await;
+        return Err(ServiceError::Timeout { timeout });
+    };
+    guard.request_id = None;
+    match response? {
+        ServerResult::CallToolResult(result) => Ok(result),
+        _ => Err(ServiceError::UnexpectedResponse),
+    }
+}
+
+/// Armed while the request is in flight; disarmed once it settles.
+struct CancelOnDrop {
+    peer: Peer<RoleClient>,
+    request_id: Option<RequestId>,
+}
+
+impl CancelOnDrop {
+    async fn cancel_within_budget(&mut self) {
+        let Some(request_id) = self.request_id.clone() else {
+            return;
+        };
+        let delivery = notify_cancelled(self.peer.clone(), request_id.clone());
+        if tokio::time::timeout(CANCEL_DELIVERY_BUDGET, delivery)
+            .await
+            .is_err()
+        {
+            tracing::warn!(
+                ?request_id,
+                "notifications/cancelled still queued after budget"
+            );
+        }
+        self.request_id = None;
+    }
+}
+
+impl Drop for CancelOnDrop {
+    fn drop(&mut self) {
+        let Some(request_id) = self.request_id.take() else {
+            return;
+        };
+        let Ok(runtime) = tokio::runtime::Handle::try_current() else {
+            tracing::warn!(
+                ?request_id,
+                "MCP call dropped outside a runtime; cancel not sent"
+            );
+            return;
+        };
+        runtime.spawn(notify_cancelled(self.peer.clone(), request_id));
+    }
+}
+
+async fn notify_cancelled(peer: Peer<RoleClient>, request_id: RequestId) {
+    let params = CancelledNotificationParam::new(
+        Some(request_id.clone()),
+        Some("client cancelled".to_owned()),
+    );
+    if let Err(e) = peer.notify_cancelled(params).await {
+        tracing::warn!(?request_id, error = %e, "notifications/cancelled not delivered");
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::sync::Arc;
+    use std::time::Duration;
+
+    use rmcp::ServiceExt;
+    use serde_json::Value;
+    use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
+
+    use super::super::{KigiClientHandler, McpClient};
+    use super::*;
+
+    type Received = Arc<parking_lot::Mutex<Vec<Value>>>;
+
+    /// Duplex MCP server: answers `initialize`, records all, answers `tools/call` only when `reply`.
+    async fn recording_service(reply: bool) -> (McpService, Received) {
+        let received = Received::default();
+        let (client_read, mut server_write) = tokio::io::duplex(64 * 1024);
+        let (server_read, client_write) = tokio::io::duplex(64 * 1024);
+        let sink = received.clone();
+        tokio::spawn(async move {
+            let mut lines = BufReader::new(server_read).lines();
+            while let Ok(Some(line)) = lines.next_line().await {
+                let msg: Value = serde_json::from_str(&line).expect("client sends JSON");
+                sink.lock().push(msg.clone());
+                let result = match msg["method"].as_str() {
+                    Some("initialize") => serde_json::json!({
+                        "protocolVersion": msg["params"]["protocolVersion"],
+                        "capabilities": { "tools": {} },
+                        "serverInfo": { "name": "recording", "version": "0.0.0" },
+                    }),
+                    Some("tools/call") if reply => serde_json::json!({
+                        "content": [{ "type": "text", "text": "done" }],
+                        "isError": false,
+                    }),
+                    _ => continue,
+                };
+                let response =
+                    serde_json::json!({ "jsonrpc": "2.0", "id": msg["id"], "result": result });
+                let encoded = format!("{response}\n");
+                server_write
+                    .write_all(encoded.as_bytes())
+                    .await
+                    .expect("client reads replies");
+            }
+        });
+        let handler = KigiClientHandler {
+            info: McpClient::make_client_info("recording"),
+            server_name: "recording".to_string(),
+            notify_tx: Arc::default(),
+        };
+        let transport = rmcp::transport::async_rw::AsyncRwTransport::<RoleClient, _, _>::new(
+            client_read,
+            client_write,
+        );
+        let service = Arc::new(handler.serve(transport).await.expect("handshake"));
+        (service, received)
+    }
+
+    fn values_at(received: &Received, method: &str, pointer: &str) -> Vec<Value> {
+        received
+            .lock()
+            .iter()
+            .filter(|m| m["method"] == method)
+            .map(|m| m.pointer(pointer).cloned().unwrap_or(Value::Null))
+            .collect()
+    }
+
+    fn call_ids(received: &Received) -> Vec<Value> {
+        values_at(received, "tools/call", "/id")
+    }
+
+    fn cancelled_ids(received: &Received) -> Vec<Value> {
+        values_at(received, "notifications/cancelled", "/params/requestId")
+    }
+
+    #[tokio::test]
+    async fn dropped_call_cancels_its_request() {
+        let (service, received) = recording_service(false).await;
+        let mut call = Box::pin(call_tool_cancel_aware(
+            &service,
+            CallToolRequestParams::new("slow"),
+            Duration::from_secs(30),
+        ));
+        let pending = tokio::time::timeout(Duration::from_millis(100), &mut call).await;
+        assert!(pending.is_err(), "server never replies");
+        drop(call);
+        tokio::time::sleep(Duration::from_millis(100)).await;
+
+        let ids = call_ids(&received);
+        assert_eq!(ids.len(), 1, "{ids:?}");
+        assert_eq!(cancelled_ids(&received), ids);
+    }
+
+    #[tokio::test]
+    async fn timed_out_call_cancels_its_request_once() {
+        let (service, received) = recording_service(false).await;
+        let round = call_tool_cancel_aware(
+            &service,
+            CallToolRequestParams::new("slow"),
+            Duration::from_millis(50),
+        )
+        .await;
+        assert!(
+            matches!(round, Err(ServiceError::Timeout { .. })),
+            "{round:?}"
+        );
+        // The HTTP timeout arm resets the transport, which drops the last service handle.
+        drop(service);
+        tokio::time::sleep(Duration::from_millis(100)).await;
+
+        let ids = call_ids(&received);
+        assert_eq!(ids.len(), 1, "{ids:?}");
+        assert_eq!(cancelled_ids(&received), ids);
+    }
+
+    #[tokio::test]
+    async fn completed_call_sends_no_cancellation() {
+        let (service, received) = recording_service(true).await;
+        let result = call_tool_cancel_aware(
+            &service,
+            CallToolRequestParams::new("fast"),
+            Duration::from_secs(5),
+        )
+        .await
+        .expect("server replies");
+        assert_eq!(result.is_error, Some(false));
+        tokio::time::sleep(Duration::from_millis(50)).await;
+        assert_eq!(cancelled_ids(&received), Vec::<Value>::new());
+    }
+}