Add early yielding for code-mode observations (#48123)

## What changed

Add an optional `preempt` signal to `CodeModeSession::execute` and `CodeModeSession::wait` so callers can end an observation early while the cell continues running and remains available for later waits.

Support the signal in the in-process runtime and across both remote transports: `operation/yield` with the negotiated `yield-observation` capability for stdio, and `YieldObservation` for gRPC. Preserve signals received before gRPC observation registration, and retain normal timeout behavior for older hosts that lack support.

## Testing

Add runtime and transport tests covering early execute and wait yields followed by successful cell completion, yields before observation registration, and suppression of yield frames for older stdio hosts.

GitOrigin-RevId: 3f8049d8f0582be6b8f0832cc03f2d974a0b8f18

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

@@ -29,6 +29,10 @@ service CodeModeHost {
   rpc Execute(ExecuteRequest) returns (stream ExecuteEvent);
   rpc Wait(WaitRequest) returns (WaitResponse);
 
+  // Ends the current observation early without stopping its cell. The
+  // corresponding Execute or Wait still returns its ordinary outcome.
+  rpc YieldObservation(YieldObservationRequest) returns (YieldObservationResponse);
+
   // Acknowledges that a canceled wait has retired before another wait starts.
   rpc CancelWait(CancelWaitRequest) returns (CancelWaitResponse);
   rpc Terminate(TerminateRequest) returns (WaitResponse);
@@ -189,6 +193,16 @@ message CancelWaitRequest {
 
 message CancelWaitResponse {}
 
+message YieldObservationRequest {
+  string session_id = 1;
+  oneof observation {
+    string execution_id = 2;
+    string wait_id = 3;
+  }
+}
+
+message YieldObservationResponse {}
+
 message TerminateRequest {
   string session_id = 1;
   string cell_id = 2;