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/Cargo.toml

@@ -19,10 +19,13 @@ codex-install-context = { workspace = true }
 codex-protocol = { workspace = true }
 codex-websocket-client = { workspace = true }
 futures = { workspace = true }
+serde_json = { workspace = true }
 tokio = { workspace = true, features = ["io-util", "macros", "net", "process", "rt", "sync", "time"] }
 tokio-tungstenite = { workspace = true }
 tokio-util = { workspace = true, features = ["rt"] }
+tonic = { workspace = true }
 tracing = { workspace = true }
+uuid = { workspace = true, features = ["v4"] }
 
 [dev-dependencies]
 pretty_assertions = { workspace = true }