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/config/src/config_toml.rs

@@ -409,10 +409,6 @@ pub struct ConfigToml {
     /// active.
     pub experimental_realtime_start_instructions: Option<String>,
 
-    /// Experimental / do not use. When set, app-server fetches thread-scoped
-    /// config from a remote service at this endpoint.
-    pub experimental_thread_config_endpoint: Option<String>,
-
     /// Removed. Former remote thread-store endpoint setting kept only so we can
     /// fail fast instead of silently falling back to local persistence.
     #[schemars(skip)]