Propagate trace context through gRPC code mode (#41017)

## What changed

- Inject W3C `traceparent` metadata into code-mode session and execution requests.
- Carry each execution's trace context on streamed tool callbacks so callback and nested-tool spans remain connected across the gRPC boundary.
- Preserve the relevant spans when execution, callback, and dispatch work moves into spawned tasks.

GitOrigin-RevId: 6482963cb9b2e2f0ea27f1345bba208b70a80d0b

codex-rs/code-mode-protocol/src/grpc/codex.code_mode.v1.proto

@@ -82,6 +82,11 @@ message ToolCall {
 
   // Starts at one and increases independently for each execution.
   uint64 sequence = 9;
+
+  // The host execution span's W3C parent context for this streamed callback.
+  // gRPC metadata is fixed when the subscription stream opens, so each call
+  // carries its own context in the message.
+  optional string traceparent = 10;
 }
 
 message CompleteToolCallRequest {