Regression sweep cell for PR 42840. Extends pricing/test_service_tier_pricing.py with balanced and flex tier billing, base-rate fallback, non-Sail extra_body.metadata shallow overwrite and the sail/ completion_window translation, adds pricing/test_responses_extra_body_pricing.py and providers/test_sail_audio_transcription.py (BUG skip, the tip answers 500), and records scripted-scenario POST bodies in the harness upstream so the tests can assert the whole outbound body. MUTATIONS.md holds the review evidence and is not for merge
