fix: propagate response_transformed flag — plugin hook output survives streaming suppression

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).

gateway/run.py

@@ -17678,7 +17678,11 @@ class GatewayRunner:
             _content_delivered = bool(
                 _sc and getattr(_sc, "final_content_delivered", False)
             )
-            if not _is_empty_sentinel and (_streamed or _previewed or _content_delivered):
+            # Plugin hooks (e.g. transform_llm_output) may have appended content
+            # after streaming finished — when the response was transformed, always
+            # send the final version so the appended content reaches the client.
+            _transformed = bool(response.get("response_transformed"))
+            if not _is_empty_sentinel and not _transformed and (_streamed or _previewed or _content_delivered):
                 logger.info(
                     "Suppressing normal final send for session %s: final delivery already confirmed (streamed=%s previewed=%s content_delivered=%s).",
                     session_key or "?",