When a transform_llm_output hook modifies final_response after streaming,
the gateway was silently discarding the transformed content because
streamed=True / content_delivered=True triggered the final-send
suppression. Three changes:
1. conversation_loop: set `_response_transformed=True` when a
transform_llm_output hook returns a non-empty string, and expose it
as `response_transformed` in the result dict.
2. gateway/run: skip the final-send suppression when
`response_transformed` is True — the transformed response must
reach the client even if streaming already sent the original text.
3. acp_adapter/server: remove `not streamed_message` guard so
final_response is always delivered (ACP path fixed separately).
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