Add turn start timestamp to turn metadata (#19473)

## Why
- Without change: MCP tool calls receive
`_meta["x-codex-turn-metadata"]` with `session_id` and `turn_id`.
- Issue: MCP servers may want the turn start timestamp to measure
internal latency relative to turn start.

## What Changed
- With change: turn metadata now includes `turn_started_at_unix_ms`,
which is propagated to MCP tool calls in
`_meta["x-codex-turn-metadata"]`.

## Verification
- `codex-rs/core/src/mcp_tool_call_tests.rs`
- `codex-rs/core/src/turn_metadata_tests.rs`
- `codex-rs/core/src/turn_timing_tests.rs`
- `codex-rs/core/tests/responses_headers.rs`
- `codex-rs/core/tests/suite/search_tool.rs`

codex-rs/core/src/tasks/mod.rs

@@ -299,10 +299,13 @@ impl Session {
         let task_kind = task.kind();
         let span_name = task.span_name();
         let started_at = Instant::now();
-        turn_context
+        let turn_started_at_unix_ms = turn_context
             .turn_timing_state
             .mark_turn_started(started_at)
             .await;
+        turn_context
+            .turn_metadata_state
+            .set_turn_started_at_unix_ms(turn_started_at_unix_ms);
         let token_usage_at_turn_start = self.total_token_usage().await.unwrap_or_default();
 
         let cancellation_token = CancellationToken::new();