Remove the experimental thread config endpoint (#39115)

## What changed

- Remove `experimental_thread_config_endpoint` from the configuration model and schema.
- Stop app-server and the in-process client from constructing a remote thread config loader from local configuration.
- Keep the thread config loader supplied at app-server startup fixed for the lifetime of the config manager.

GitOrigin-RevId: e7907fee2d631e7ecf89ec6bac8f22bc2c221350

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

@@ -287,7 +287,6 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         experimental_realtime_ws_backend_prompt: None,
         experimental_realtime_ws_startup_context: None,
         experimental_realtime_start_instructions: None,
-        experimental_thread_config_endpoint: None,
         experimental_thread_store: ThreadStoreConfig::Local,
         forced_chatgpt_workspace_id: None,
         forced_login_method: None,