Support remote code-mode hosts in app-server (#35098)

## What changed

- Add `--code-mode-host ws://...` and `wss://...` support to `codex app-server`, gated by the `code_mode_host` feature. When omitted, app-server continues to start a local host.
- Share one remote WebSocket connection across the process's threads, using the configured HTTP client's proxy and TLS policy and preserving the existing framed host protocol.
- Reject invalid host URLs, bound WebSocket frame sizes, close connections cleanly, and return an error when a connection exceeds 1,024 pending delegate calls without disconnecting it.

## Testing

- Cover CLI validation, WebSocket protocol execution and shutdown, connection sharing across app-server threads, and delegate-call capacity recovery.

GitOrigin-RevId: 715e82d4d9db1e7e2f91b754a777dcab504e2ae4

codex-rs/core/src/thread_manager_tests.rs

@@ -428,12 +428,14 @@ async fn code_mode_session_provider_is_shared_across_threads() {
     config.cwd = config.codex_home.abs();
     std::fs::create_dir_all(&config.codex_home).expect("create codex home");
 
+    let provider: Arc<dyn CodeModeSessionProvider> = Arc::new(InProcessCodeModeSessionProvider);
     let manager = ThreadManager::with_models_provider_and_home_for_tests(
         CodexAuth::from_api_key("dummy"),
         config.model_provider.clone(),
         config.codex_home.to_path_buf(),
         Arc::new(codex_exec_server::EnvironmentManager::default_for_tests()),
-    );
+    )
+    .with_code_mode_session_provider(Arc::clone(&provider));
     let first = manager
         .start_thread(StartThreadOptions::new(config.clone()))
         .await
@@ -456,6 +458,7 @@ async fn code_mode_session_provider_is_shared_across_threads() {
         .code_mode_service
         .session_provider();
     assert!(Arc::ptr_eq(&first_provider, &second_provider));
+    assert!(Arc::ptr_eq(&first_provider, &provider));
     assert!(Arc::ptr_eq(
         &first_provider,
         &manager.state.code_mode_session_provider