PR #29119 dropped the 'not streamed_message' guard unconditionally so
that plugin-transformed responses (transform_llm_output hook) would
reach ACP clients. That regressed test_prompt_does_not_duplicate_streamed_final_message:
when no transform happened, the streamed text was re-sent as a duplicate
final delivery.
Tighten the condition to mirror the gateway side: deliver after streaming
only when response_transformed=True. Otherwise keep the old guard.
Adds test_prompt_delivers_transformed_response_after_streaming so the
transformed path stays covered.
60d20a37c9