fix(responses): provenance-gate foreign turns (Pi transform-messages)

Cross-provider audit R5 + R2-residual: BackendToolCall items round-trip
as their typed shapes with provider-issued ids (grok x_search
CustomToolCall, web_search/code_interpreter calls) and Reasoning items
carry model-bound encrypted payloads — replaying either to a DIFFERENT
Responses target names undeclared tools / undecryptable material → 400.

New transform_items_for_responses pre-pass: each [Reasoning|
BackendToolCall]* Assistant run carries provenance in
AssistantItem.model_id; on confirmed mismatch with the request's target
model, Reasoning siblings are dropped and BackendToolCall demoted to the
same text summary the Messages/ChatCompletions builders already emit.
Same-model and unknown-provenance turns stay byte-verbatim (KV-cache
prefix stability preserved). The legacy-upgrade round-trip test now
models the same-model continuation it always described.

Part 5 of the cross-provider replay audit.

Verified: sampling-types 290 + downstream 5794 green, clippy clean.

crates/codegen/kigi-shell/tests/test_sampling_client.rs

@@ -472,7 +472,12 @@ async fn responses_upgrade_roundtrips_reconstructed_reasoning_as_typed_input() {
             "\n",
             r#"{"type":"user","content":[{"type":"text","text":"q1"}]}"#,
             "\n",
-            r#"{"type":"assistant","content":"a1","reasoning":{"text":"legacy kigi reasoning","encrypted":"ENC_BLOB_xyz","id":"rs_kigibuild_legacy"},"model_id":"kigi"}"#,
+            // model_id matches the test client's request model: this test
+            // covers the SAME-MODEL continuation (the byte-stable
+            // SGLang-prefix path). A mismatched model_id is the provenance
+            // gate's territory (`transform_items_for_responses`) and drops
+            // the reasoning by design.
+            r#"{"type":"assistant","content":"a1","reasoning":{"text":"legacy kigi reasoning","encrypted":"ENC_BLOB_xyz","id":"rs_kigibuild_legacy"},"model_id":"test-model"}"#,
             "\n",
         ),
     )