Add configurable gating for the sleep tool (#41243)

## What changed

- Add a stable `sleep_tool` feature that can enable or disable registration of the built-in sleep tool independently of the clock tool.
- Support structured `features.sleep_tool` configuration with `model_driven` and `always_on` modes. The default `model_driven` mode preserves the existing model and `current_time_reminder` behavior, while `always_on` registers sleep whenever the feature is enabled.
- Preserve nested sleep-tool configuration across CLI overrides, config merges, and feature toggles, and expose the settings in the generated config schema.

## Testing

- Cover feature-map overrides, both selection modes, legacy clock settings, config merging and editing, and invalid mode rejection.

GitOrigin-RevId: 18eadc582f8a4445958c29d063e5be495276703e

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

@@ -304,6 +304,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         token_budget: None,
         rollout_budget: None,
         current_time_reminder: None,
+        sleep_tool_mode: Default::default(),
         features: Default::default(),
         suppress_unstable_features_warning: false,
         active_project: ProjectConfig { trust_level: None },