feat(config) experimental_request_user_input toggle (#24541)

## Summary
Experimental flag to allow toggling `request_user_input`:

```
tools.experimental_request_user_input = false
```

## Testing
- [x] Added unit tests

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

@@ -265,6 +265,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         forced_login_method: None,
         web_search_mode: Constrained::allow_any(WebSearchMode::Disabled),
         web_search_config: None,
+        experimental_request_user_input_enabled: true,
         use_experimental_unified_exec_tool: false,
         background_terminal_max_timeout: 300_000,
         ghost_snapshot: GhostSnapshotConfig::default(),