Add execution control request contracts

engine.go

@@ -60,6 +60,10 @@ var (
 	// A [CapacityError] reports the selected backend ID. It is not an invalid
 	// request, an LLM or provider rate-limit response, or ErrLLMGenerate.
 	ErrCapacityExceeded = errors.New("backend capacity exceeded")
+	// ErrOperationTimeout identifies expiration of the timeout selected by
+	// RunRequest.OperationTimeout. Such errors also match context.DeadlineExceeded
+	// and retain their underlying operation and collaborator identities.
+	ErrOperationTimeout = errors.New("operation timeout exceeded")
 	// ErrLLMGenerate identifies a model-client failure or a nil successful
 	// response. A built-in OpenAI-compatible non-2xx response is available as a
 	// [GenerationError]. Errors returned by an injected LLMClient remain