Track code-mode cell lifecycle and result buffering time (#55653)

## What changed

- Add `CodeModeCellTiming` notifications for yielded executions, terminal waits, and unavailable cells, with thread, call, and cell identifiers.
- Measure how long completed results remain buffered before retrieval and carry `result_buffered_for` through the runtime, stdio protocol, and gRPC response metadata. Immediate delivery reports zero; waits without a completed result report no buffering measurement.
- Default missing buffering data to zero for compatibility with older hosts.

## Testing

Extend runtime tests to verify buffering duration with a paused clock and retrieval through termination. Add gRPC conversion tests for nanosecond precision, optional metadata, and the distinction between immediate delivery and no completed result.

GitOrigin-RevId: 60c0c5a3affa69ac3779bf1812fca50f5748af76

codex-rs/code-mode-protocol/src/runtime_tests.rs

@@ -33,6 +33,7 @@ fn code_mode_host_duration_survives_runtime_and_stdio_serialization() {
         RuntimeResponse::Result {
             cell_id: CellId::new("completed-cell".to_string()),
             content_items,
+            result_buffered_for: Duration::ZERO,
             error_text: Some("execution failed".to_string()),
             code_mode_host_duration: None,
         },