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-runtime/src/session_runtime/tests.rs

@@ -83,6 +83,7 @@ async fn reports_cell_actor_panics_to_the_owner() {
             content_items: vec![OutputItem::Text {
                 text: "done".to_string(),
             }],
+            result_buffered_for: Duration::ZERO,
             error_text: None,
         })
     );
@@ -107,6 +108,7 @@ async fn termination_rejects_a_waiting_store_commit_before_the_next_cell_can_loa
         content_items: vec![OutputItem::Text {
             text: "uncommitted output".to_string(),
         }],
+        result_buffered_for: Duration::ZERO,
         error_text: None,
     };
 
@@ -163,6 +165,7 @@ async fn termination_rejects_a_waiting_store_commit_before_the_next_cell_can_loa
             content_items: vec![OutputItem::Text {
                 text: "undefined".to_string(),
             }],
+            result_buffered_for: Duration::ZERO,
             error_text: None,
         })
     );

codex-rs/code-mode-runtime/src/session_runtime/types.rs

@@ -66,6 +66,8 @@ pub(crate) enum CellEvent {
     Completed {
         content_items: Vec<OutputItem>,
         error_text: Option<String>,
+        /// Time the completed result spent waiting for an observer.
+        result_buffered_for: Duration,
     },
     Terminated {
         content_items: Vec<OutputItem>,