Configure the default code-mode exec yield timeout (#37352)

## What changed

- Add `features.code_mode.default_exec_yield_time_ms`, defaulting to 30 seconds.
- Apply the configured default to code-mode `exec` calls that omit `yield_time_ms` and reflect it in the tool description.
- Remove the `code_mode_buffered_exec` compatibility feature flag.

## Testing

- Cover config loading and tool descriptions for both default and custom yield timeouts.

GitOrigin-RevId: 581ffc737b04721c7b5fc81260b1da1cf7d87523

codex-rs/core/src/session/session.rs

@@ -1204,7 +1204,7 @@ impl Session {
                 executed_tool_calls,
                 code_mode_service: crate::tools::code_mode::CodeModeService::new(
                     Arc::clone(&code_mode_session_provider),
-                    &config.features,
+                    &config.code_mode,
                 ),
                 tool_search_handler_cache: Default::default(),
                 turn_environments: Arc::clone(&turn_environments),