Add buffered code-mode exec yields (#34441)

## What changed

- Add the experimental `code_mode_buffered_exec` feature.
- When enabled, default code-mode `exec` calls without an explicit
  `yield_time_ms` to 30 seconds instead of 10 seconds.
- Report the effective default in the model-visible `exec` tool description.

## Testing

- Verify that the generated tool description advertises the 30-second default
  when the feature is enabled.

GitOrigin-RevId: 0eea4d0105137f09050ce31aad18c25c79df6b2a

codex-rs/features/src/lib.rs

@@ -92,6 +92,8 @@ pub enum Feature {
     // Experimental
     /// Enable JavaScript code mode backed by the in-process V8 runtime.
     CodeMode,
+    /// Use a 30-second default yield timeout for code mode exec calls.
+    CodeModeBufferedExec,
     /// Run JavaScript code mode in the standalone host process.
     CodeModeHost,
     /// Restrict model-visible tools to code mode entrypoints (`exec`, `wait`).
@@ -859,6 +861,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::CodeModeBufferedExec,
+        key: "code_mode_buffered_exec",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::CodeModeHost,
         key: "code_mode_host",