Allow disabling the update_plan tool (#35054)

## What changed

- Add a default-on `tools.update_plan.enabled` configuration option.
- Omit `update_plan` from the visible and registered tool sets when the option is disabled.

## Testing

- Cover configuration resolution and tool registration for the disabled setting.

GitOrigin-RevId: c13aa463a6911956fca9f0ef5b74841b543798c4

codex-rs/thread-manager-sample/src/main.rs

@@ -294,6 +294,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         web_search_mode: Constrained::allow_any(WebSearchMode::Disabled),
         web_search_config: None,
         experimental_request_user_input_enabled: true,
+        update_plan_enabled: true,
         code_mode: Default::default(),
         use_experimental_unified_exec_tool: false,
         background_terminal_max_timeout: 300_000,