Add gRPC-backed code-mode sessions (#38041)

## What changed

- Add `GrpcCodeModeSessionProvider` for opening code-mode sessions over HTTP/2 or an existing `tonic` channel.
- Support execution, waiting, termination, per-session limits, cell-closure callbacks, and graceful shutdown over the gRPC protocol.
- Bound transport waits and error messages, validate host identifiers and responses, and clean up abandoned executions and observers.

## Testing

- Add end-to-end TCP tests covering session persistence, cancellation, concurrent waits, shutdown, cell cleanup, and independent yield limits.
- Add unit coverage for protocol conversion, deadlines, and session lifecycle state.

GitOrigin-RevId: d4729ce608ad4b42a99744b07e1f230e46cb24ec

codex-rs/code-mode/src/remote_session.rs

@@ -32,7 +32,7 @@ use crate::NoopCodeModeSessionDelegate;
 
 mod connection;
 
-type ShutdownResultReceiver = watch::Receiver<Option<Result<(), String>>>;
+pub(crate) type ShutdownResultReceiver = watch::Receiver<Option<Result<(), String>>>;
 
 /// Creates code-mode sessions backed by one lazily spawned process host.
 pub struct ProcessOwnedCodeModeSessionProvider {
@@ -547,7 +547,7 @@ enum ShutdownAction {
     Close(SessionBinding),
 }
 
-async fn wait_for_watch<T>(
+pub(crate) async fn wait_for_watch<T>(
     mut result_rx: watch::Receiver<Option<Result<T, String>>>,
 ) -> Result<T, String>
 where