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

agent/conversation_loop.py

@@ -4047,7 +4047,6 @@ def run_conversation(
                 model=agent.model,
                 platform=getattr(agent, "platform", None) or "",
             )
-            _response_transformed = False
             for _hook_result in _transform_results:
                 if isinstance(_hook_result, str) and _hook_result:
                     final_response = _hook_result