…s 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).

…turn dict

run_sync() cherry-picks fields from the run_conversation result dict into
a new response dict for the gateway. response_transformed was missing from
the cherry-pick list, so the gateway always saw it as False and suppressed
the final send even though a transform_llm_output hook had modified the content.

… response_transformed

When a transform_llm_output hook appends content after streaming, the previous
fix skipped the final-send suppression which caused the full response to be
sent as a NEW message (duplicate). Instead, edit the existing streamed message
in-place to append the transformed content, then set already_sent=True.

Added stream_consumer.message_id and .accumulated_text public properties.

…ming

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