Run code mode exclusively through the standalone host (#36217)

## What changed

- Move the V8 implementation into a dedicated `codex-code-mode-runtime` crate used by `codex-code-mode-host`, removing the embedded runtime fallback from the Codex process.
- Resolve the host executable from the active installation layout and check its availability before selecting tools.
- Fall back to direct tools with a one-time warning when optional code mode is unavailable. Keep `code_mode_only` and `disable_in_process_fallback` configurations fail-closed.

## Testing

- Cover host discovery for standalone and package layouts, including missing hosts and symlinks.
- Verify direct-tool fallback, one-time warnings, and fail-closed code-mode-only behavior.

GitOrigin-RevId: 5aa3c6f1db148b2231fc24089a2ee0e2b00dbddb

codex-rs/core/src/thread_manager_tests.rs

@@ -429,7 +429,7 @@ 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 provider: Arc<dyn CodeModeSessionProvider> = Arc::new(DisabledCodeModeSessionProvider);
     let manager = ThreadManager::with_models_provider_and_home_for_tests(
         CodexAuth::from_api_key("dummy"),
         config.model_provider.clone(),