[GA-M2][testing] Add offline route replay fixtures for cost and quality guardrails

[Xunzhuo]

Updated from the GA issue triage. This issue is intended to be directly assignable.

Goal

Test route replay behavior when cheaper routing conflicts with verifier or quality guardrails.

Deliverables

• Fixture where candidate policy reduces cost without quality regression.
• Fixture where candidate policy is rejected due to verifier weakness or failure.
• Fixture where self-hosted preference applies only to safe request classes.

Acceptance Criteria

• Replay tests verify both accepted and rejected policy changes.
• Quality guardrails can override cost savings.
• Reports show why a candidate policy was rejected.

Dependencies

• [GA-M2][evaluation] Implement offline SR route-policy replay over ga-eval traces #20