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