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/code-mode-protocol/src/lib.rs

@@ -38,6 +38,7 @@ pub use session::CodeModeSessionDelegate;
 pub use session::CodeModeSessionProvider;
 pub use session::CodeModeSessionProviderFuture;
 pub use session::CodeModeSessionResultFuture;
+pub use session::NoopCodeModeSessionDelegate;
 pub use session::NotificationFuture;
 pub use session::StartedCell;
 pub use session::ToolInvocationFuture;

codex-rs/code-mode-protocol/src/session.rs

@@ -104,6 +104,34 @@ pub trait CodeModeSessionDelegate: Send + Sync {
     fn cell_closed(&self, cell_id: &CellId);
 }
 
+/// A session delegate for clients that do not expose nested tools or notifications.
+pub struct NoopCodeModeSessionDelegate;
+
+impl CodeModeSessionDelegate for NoopCodeModeSessionDelegate {
+    fn invoke_tool<'a>(
+        &'a self,
+        _invocation: CodeModeNestedToolCall,
+        cancellation_token: CancellationToken,
+    ) -> ToolInvocationFuture<'a> {
+        Box::pin(async move {
+            cancellation_token.cancelled().await;
+            Err("code mode nested tools are unavailable".to_string())
+        })
+    }
+
+    fn notify<'a>(
+        &'a self,
+        _call_id: String,
+        _cell_id: CellId,
+        _text: String,
+        _cancellation_token: CancellationToken,
+    ) -> NotificationFuture<'a> {
+        Box::pin(async { Ok(()) })
+    }
+
+    fn cell_closed(&self, _cell_id: &CellId) {}
+}
+
 /// A durable code-mode session owned by one Codex thread.
 ///
 /// Cells executed in the same session share stored values. Separate sessions
@@ -127,6 +155,11 @@ pub trait CodeModeSession: Send + Sync {
 /// Implementations may share a remote host process across all sessions created
 /// by one provider.
 pub trait CodeModeSessionProvider: Send + Sync {
+    /// Reports whether this provider can execute code without starting its host.
+    fn availability(&self) -> Result<(), String> {
+        Ok(())
+    }
+
     fn create_session<'a>(
         &'a self,
         delegate: Arc<dyn CodeModeSessionDelegate>,