Add Anthropic platform: wire-served metadata via listing dialect (provider 2)

The 5th registry row: id "anthropic", ANTHROPIC_API_KEY > auth.json
"anthropic" scope, api.anthropic.com/v1 with KIGI_ANTHROPIC_BASE_URL
override, Messages dialect. Two new spec dimensions most future rows
reuse: ListingDialect (Anthropic's /v1/models wants x-api-key +
anthropic-version headers, ?limit=1000, and its own response shape) and
PlatformKeyHeader (Bearer vs x-api-key across listing/validation/
inference, with auth_scheme stamped onto entries).

The 2026 Anthropic listing serves real metadata: the adapter maps
max_input_tokens, per-level effort capabilities (low..max as the menu,
xhigh/max distinct), thinking/image flags — and enrichment fills only
genuine wire gaps (e2e pins wire-1M beating enrichment, and a zero
context filled to 200k).

Two review-confirmed defects fixed red-green:
- Output caps were dropped at three layers, so every sub-128K-output
  model (64k Haiku, legacy models) would 400 on EVERY request against
  the sampler's 128K max_tokens default. Wire max_tokens and enrichment
  limit.output now flow to entry.max_completion_tokens.
- An explicit wire effort-decline was indistinguishable from wire
  silence, letting enrichment inject effort menus pre-4.6 models reject
  (adaptive thinking 400). The adapter now emits a decline sentinel
  (support:false) that enrichment respects — proven end to end.

Also: the Messages client now sends anthropic-version (previously never
sent — real api.anthropic.com rejects such requests; pinned across all
three scheme/backend quadrants), key validation builds per-key-header
requests, missing listing data fails fast, empty-id ghosts drop with a
warning, kimi-import recognizes api.anthropic.com as built-in
automatically.

crates/codegen/kigi-sampler/src/client.rs

@@ -371,6 +371,14 @@ impl SamplingClient {
                         )
                     })?;
                     headers.insert(HeaderName::from_static("x-api-key"), header_value);
+                    if config.api_backend == kigi_sampling_types::ApiBackend::Messages {
+                        // The real Anthropic Messages wire rejects requests
+                        // without this; compatible endpoints ignore it.
+                        headers.insert(
+                            HeaderName::from_static("anthropic-version"),
+                            HeaderValue::from_static(kigi_sampling_types::ANTHROPIC_VERSION),
+                        );
+                    }
                 }
                 AuthScheme::Bearer => {
                     let bearer = format!("Bearer {}", api_key);
@@ -1890,6 +1898,41 @@ mod tests {
         }
     }
 
+    /// The real Anthropic Messages wire rejects requests without
+    /// `anthropic-version`; the XApiKey+Messages client must carry it in its
+    /// default headers, and Bearer/ChatCompletions clients must NOT.
+    #[test]
+    fn x_api_key_messages_client_sends_anthropic_version() {
+        let mut config = minimal_config();
+        config.auth_scheme = AuthScheme::XApiKey;
+        config.api_backend = ApiBackend::Messages;
+        let client = SamplingClient::new(config).expect("client builds");
+        assert_eq!(
+            client
+                .default_headers
+                .get("anthropic-version")
+                .and_then(|v| v.to_str().ok()),
+            Some(kigi_sampling_types::ANTHROPIC_VERSION)
+        );
+        assert!(client.default_headers.get("x-api-key").is_some());
+
+        let bearer = SamplingClient::new(minimal_config()).expect("client builds");
+        assert!(
+            bearer.default_headers.get("anthropic-version").is_none(),
+            "non-anthropic clients must not grow the header"
+        );
+        let mut x_api_chat = minimal_config();
+        x_api_chat.auth_scheme = AuthScheme::XApiKey;
+        let x_api_chat = SamplingClient::new(x_api_chat).expect("client builds");
+        assert!(
+            x_api_chat
+                .default_headers
+                .get("anthropic-version")
+                .is_none(),
+            "XApiKey without the Messages backend must not grow the header"
+        );
+    }
+
     /// Verify the serialized shape of StreamingChatRequest matches the
     /// expected wire format: all ChatCompletionRequest fields flattened at
     /// top level, plus `stream: true` and `stream_options.include_usage: true`.