Turn-state sticky routing per turn (#9332)

- capture the header from SSE/WS handshakes, store it per
ModelClientSession using `Oncelock`, echo it on turn-scoped requests,
and add SSE+WS integration tests for within-turn persistence +
cross-turn reset.

- keep `x-codex-turn-state` sticky within a user turn to maintain
routing continuity for retries/tool follow-ups.

codex-rs/codex-api/tests/sse_end_to_end.rs

@@ -129,6 +129,7 @@ async fn responses_stream_parses_items_and_completed_end_to_end() -> Result<()>
             serde_json::json!({"echo": true}),
             HeaderMap::new(),
             Compression::None,
+            None,
         )
         .await?;
 
@@ -198,6 +199,7 @@ async fn responses_stream_aggregates_output_text_deltas() -> Result<()> {
             serde_json::json!({"echo": true}),
             HeaderMap::new(),
             Compression::None,
+            None,
         )
         .await?;