Fix _GatedLLM.completion override after _return_metrics removal

The `_return_metrics` parameter was removed from LLM/TestLLM.completion in
2bedbd8 (removed_in 1.29.0, on this release branch), but
tests/agent_server/test_goal_loop.py's _GatedLLM override (added in #3770)
still declared it as a positional param and forwarded it to
super().completion(), which pyright flagged as an incompatible override
and a 5-positional call against the new 4-positional signature. Drop
_return_metrics to match.

Author: VascoSch92

tests/agent_server/test_goal_loop.py

@@ -43,7 +43,6 @@ def completion(
         self,
         messages,
         tools=None,
-        _return_metrics=False,
         add_security_risk_prediction=False,
         on_token=None,
         **kwargs,
@@ -53,7 +52,6 @@ def completion(
         return super().completion(
             messages,
             tools,
-            _return_metrics,
             add_security_risk_prediction,
             on_token,
             **kwargs,