Report code mode host request durations (#41452)

## Why

Code mode wall time should measure the host operation itself, without including
client-side response delays or idle time between requests.

## What changed

- Measure each execute, wait, and terminate request in the code mode host.
- Carry the duration through the stdio and gRPC protocols and use it for
  model-visible wall time.
- Emit a structured `codex.code_mode.host_timing` event correlated with the
  conversation, turn, tool call, and cell.

## Testing

- Cover successful and failed execution timing, delayed response reads,
  repeated waits, termination, and missing cells across stdio and gRPC.
- Verify timing survives protocol serialization and is reflected in app-server
  model output and structured telemetry.

GitOrigin-RevId: d24af30c3fc5820521b4beba1f9970714dad6482

codex-rs/code-mode-runtime/src/service.rs

@@ -347,6 +347,7 @@ fn runtime_response(
         runtime::CellEvent::Yielded { content_items } => Ok(RuntimeResponse::Yielded {
             cell_id: cell_id.clone(),
             content_items: content_items.into_iter().map(output_item).collect(),
+            code_mode_host_duration: None,
         }),
         runtime::CellEvent::Completed {
             content_items,
@@ -355,10 +356,12 @@ fn runtime_response(
             cell_id: cell_id.clone(),
             content_items: content_items.into_iter().map(output_item).collect(),
             error_text,
+            code_mode_host_duration: None,
         }),
         runtime::CellEvent::Terminated { content_items } => Ok(RuntimeResponse::Terminated {
             cell_id: cell_id.clone(),
             content_items: content_items.into_iter().map(output_item).collect(),
+            code_mode_host_duration: None,
         }),
         runtime::CellEvent::Pending { .. } => {
             Err("cell returned a pending frontier unexpectedly".to_string())
@@ -391,6 +394,7 @@ fn missing_cell_response(cell_id: CellId) -> RuntimeResponse {
         error_text: Some(format!("exec cell {cell_id} not found")),
         cell_id,
         content_items: Vec::new(),
+        code_mode_host_duration: None,
     }
 }