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-protocol/src/grpc/codex.code_mode.v1.proto

@@ -134,7 +134,8 @@ message CellClosed {
   string execution_id = 1;
   string cell_id = 2;
 
-  // All lower sequences must be observed before the client retires the cell.
+  // Last tool-call sequence issued before closure. Clients may retire the cell
+  // immediately and reject tool calls delivered after its closure.
   uint64 final_tool_call_sequence = 3;
 }
 

codex-rs/code-mode-protocol/src/grpc/mod.rs

@@ -3,3 +3,5 @@ pub use code_mode_proto::codex::code_mode::v1::*;
 
 #[cfg(not(codex_bazel))]
 tonic::include_proto!("codex.code_mode.v1");
+
+pub const MAX_IDENTIFIER_BYTES: usize = 256;