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-host/src/grpc/robustness_tests.rs

@@ -18,6 +18,7 @@ use tonic::Request;
 use tonic::Status;
 use uuid::Uuid;
 
+use super::ExecutionAdmission;
 use super::GrpcCodeModeHost;
 use super::tests::execute_events;
 use super::tests::execute_request;
@@ -125,6 +126,31 @@ async fn rejects_oversized_identifiers_tool_metadata_and_subscription_filters()
     assert!(host.state.session(&session_id).is_ok());
 }
 
+#[tokio::test]
+async fn dropping_execution_before_admission_releases_its_reservation() {
+    let host = GrpcCodeModeHost::new();
+    let (session_id, _events) = open_session(&host).await;
+    let session = host.state.session(&session_id).expect("open session");
+    let execution_id = "execution-abandoned-before-admission".to_string();
+    session
+        .reserve_execution(&execution_id)
+        .expect("reserve execution");
+
+    drop(ExecutionAdmission {
+        session: Arc::clone(&session),
+        execution_id: Some(execution_id.clone()),
+    });
+
+    let error = session
+        .admit_execution(
+            execution_id,
+            "cell".to_string(),
+            host.state.cell_permit().expect("reserve cell permit"),
+        )
+        .expect_err("abandoned execution must not admit a runtime cell");
+    assert_eq!(error.code(), Code::Cancelled);
+}
+
 #[tokio::test]
 async fn dropping_an_unread_buffered_execution_outcome_retires_its_cell() {
     let host = GrpcCodeModeHost::new();

codex-rs/code-mode-host/src/grpc/validation.rs

@@ -2,7 +2,7 @@ use codex_code_mode_protocol::grpc as proto;
 use tonic::Status;
 use uuid::Uuid;
 
-pub(super) const MAX_IDENTIFIER_BYTES: usize = 256;
+pub(super) use codex_code_mode_protocol::grpc::MAX_IDENTIFIER_BYTES;
 pub(super) const MAX_TOOL_FILTERS: usize = 64;
 pub(super) const MAX_TOOL_DEFINITIONS: usize = 1_024;
 pub(super) const MAX_TOOL_DESCRIPTION_BYTES: usize = 16 * 1_024;