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/src/remote_session/connection/driver/cell_ids.rs

@@ -78,11 +78,13 @@ pub(super) fn public_runtime_response(
         RuntimeResponse::Result {
             cell_id,
             content_items,
+            result_buffered_for,
             error_text,
             code_mode_host_duration,
         } => RuntimeResponse::Result {
             cell_id: public_cell_id_from_protocol(generation, &cell_id),
             content_items,
+            result_buffered_for,
             error_text,
             code_mode_host_duration,
         },