fix(gateway): set empty dict instead of None for request_overrides

[joesaiw1208-stack]

When service_tier is empty, _build_turn_route() was setting request_overrides to None, which would later override the dict initialized in AIAgent.init. This caused an AttributeError when code tried to call .get() on None in run_agent.py line 5522.

The fix is to use {} instead of None, consistent with how request_overrides is initialized in AIAgent.init.

[alt-glitch]

Related to #7281 — competing fix for the same AttributeError crash when request_overrides is None (reported in #7259).

[teknium1]

Automated hermes-sweeper review: this PR's requested fix is already implemented on current main.

Evidence:

• PR fix(gateway): set empty dict instead of None for request_overrides #7320 changes gateway/run.py so _resolve_turn_agent_config() returns {} instead of None for request_overrides when no service_tier applies.
• Current main already does that in gateway/run.py:3612, and also normalizes unsupported fast-mode overrides with overrides or {} at gateway/run.py:3619.
• Current main also guards cached-agent turn updates with agent.request_overrides = turn_route.get("request_overrides") or {} at gateway/run.py:16492.
• Regression coverage exists in tests/gateway/test_fast_command.py:129, asserting the unsupported-model path returns {}.
• The equivalent fix landed in commit e4b69bf149290eaf423e658cf285fb024262b2d4 (fix(gateway): guard against None request_overrides in _build_api_kwargs).

Thanks for the report and patch. This overlaps with the related competing fix mentioned in the thread (#7281 / #7259), so the behavioral guarantee is now covered on main.