[apps] Add apps MCP path override (#20231)

Summary

- Add `[features.apps_mcp_path_override]` config with a `path` field for
overriding only the built-in apps MCP path.
- Keep existing host/base URL derivation unchanged and append the
configured path after that base.
- Regenerate the config schema with the custom feature-config case.

Test Plan

- Not run for latest revision; only `just fmt` and `just
write-config-schema` were run.
- Earlier revision: `cargo test -p codex-features`
- Earlier revision: `cargo test -p codex-mcp`

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

@@ -237,6 +237,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         model_catalog: None,
         model_verbosity: None,
         chatgpt_base_url: "https://chatgpt.com/backend-api/".to_string(),
+        apps_mcp_path_override: None,
         realtime_audio: RealtimeAudioConfig::default(),
         experimental_realtime_ws_base_url: None,
         experimental_realtime_ws_model: None,