Add execution control request contracts

internal/usecase/runner.go

@@ -39,6 +39,7 @@ var (
 	ErrPromptRender     = errors.New("failed to render prompt")
 	ErrLLMGenerate      = errors.New("failed to generate output")
 	ErrValidation       = errors.New("failed to validate output")
+	ErrOperationTimeout = errors.New("operation timeout exceeded")
 )
 
 // Runner executes the Promptkit core use case.