fix(acp): only deliver final_response after streaming when transformed

PR #29119 dropped the 'not streamed_message' guard unconditionally so
that plugin-transformed responses (transform_llm_output hook) would
reach ACP clients. That regressed test_prompt_does_not_duplicate_streamed_final_message:
when no transform happened, the streamed text was re-sent as a duplicate
final delivery.

Tighten the condition to mirror the gateway side: deliver after streaming
only when response_transformed=True. Otherwise keep the old guard.

Adds test_prompt_delivers_transformed_response_after_streaming so the
transformed path stays covered.

acp_adapter/server.py

@@ -1534,9 +1534,11 @@ class HermesACPAgent(acp.Agent):
                 )
             except Exception:
                 logger.debug("Failed to auto-title ACP session %s", session_id, exc_info=True)
-        if final_response and conn:
-            # Always deliver the final response — plugins may have transformed
-            # it after streaming finished (e.g. transform_llm_output hook).
+        if final_response and conn and (not streamed_message or result.get("response_transformed")):
+            # Deliver the final response when streaming did not already send it,
+            # or when a plugin hook transformed the response after streaming
+            # finished (e.g. transform_llm_output) — otherwise the appended /
+            # rewritten text never reaches the client.
             update = acp.update_agent_message_text(final_response)
             await conn.session_update(session_id, update)