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-host/src/host_tests.rs

@@ -7,6 +7,7 @@ use std::sync::atomic::AtomicBool;
 use std::task::Context;
 use std::task::Poll;
 use std::time::Duration;
+use std::time::Instant;
 
 use codex_code_mode_protocol::CodeModeSessionCellExecutionLimits;
 use codex_code_mode_protocol::host::Capability;
@@ -594,6 +595,7 @@ async fn active_cell_limit_rejects_execute_without_disconnecting() {
                 request: execute_request("text(\"hello\");"),
             },
             CancellationToken::new(),
+            Instant::now(),
         )
         .await;