fix(gateway): guard against None request_overrides in _build_api_kwargs

[kshitijk4poor]

Summary

Salvage of #7305 by @konsisumer — normalizes request_overrides from None to {} at the source in gateway/run.py so downstream code never encounters NoneType.

Closes #7215, closes #7259, closes #7277

Background

Commit 970192f (fast mode support) introduced request_overrides plumbing in the gateway. Three sites in _resolve_turn_agent_config could produce None:

1. No service tier configured → route["request_overrides"] = None
2. resolve_fast_mode_overrides() raises → overrides = None
3. Cached agent path directly assigns turn_route.get("request_overrides") which can be None

This caused AttributeError: 'NoneType' object has no attribute 'get' on every gateway message for users without fast mode configured.

The acute crash was already mitigated by PR #7350 (added or {} at the access site in _build_api_kwargs). This PR fixes the root cause — the gateway now always produces {} instead of None, preventing any future consumer from hitting the same bug.

Changes

• gateway/run.py: Three sites in _resolve_turn_agent_config normalized from None to {}
• tests/gateway/test_fast_command.py: Updated assertion from is None to == {}
• tests/run_agent/test_provider_parity.py: New test for _build_api_kwargs with request_overrides = None

All 4 targeted tests pass. Clean cherry-pick, no conflicts (only 19 commits behind main).

[github-actions]

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Files:

hermes_cli/setup.py

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