perf(gateway): reuse loaded turn config for timestamp check

Re-derivation of PR #65645 onto current main: _build_gateway_agent_history
already runs inside a turn whose config was loaded once into
ctx.user_config; re-reading config from disk via _load_gateway_config()
per turn is redundant. Reuse the loaded turn config.

gateway/run.py

@@ -4965,7 +4965,7 @@ class TurnRunner:
         agent_history, observed_group_context = _build_gateway_agent_history(
             ctx.history,
             channel_prompt=ctx.channel_prompt,
-            inject_timestamps=_message_timestamps_enabled(_load_gateway_config()),
+            inject_timestamps=_message_timestamps_enabled(ctx.user_config),
         )
 
         # FTS write-corruption guard (#50502): when message persistence