test(gateway): regression for plugin-transformed response after streaming

Adds a test that fails without the gateway fix, exercising the
response_transformed=True branch in _finalize_response: a streamed
response whose final text was modified by a transform_llm_output
plugin hook must be edit_message'd in place (not duplicate-sent),
with already_sent=True so the normal final-send is skipped.

Also drops two minor leftovers from the salvaged PR #29119:

  * accumulated_text property on GatewayStreamConsumer (unused)
  * duplicate _response_transformed=False inside the hook try block

gateway/stream_consumer.py

@@ -197,11 +197,6 @@ class GatewayStreamConsumer:
         """The Discord/chat message ID of the last-sent or edited message."""
         return self._message_id
 
-    @property
-    def accumulated_text(self) -> str:
-        """The accumulated streamed text (without think-block content)."""
-        return self._accumulated
-
     @property
     def final_content_delivered(self) -> bool:
         """True when the final response content reached the user, even if