Summary

• Persist one request-scoped effective is_free decision and consume it for billable cost zeroing and usage metadata instead of re-evaluating model classification during accounting.
• Make isFreeModel() classify intrinsic free models only; after provider resolution, a selected active experiment route is folded into is_free for that request.
• Prevent stale positive experiment membership from allowing anonymous paid fallback traffic, and only log free-model quota after provider resolution accepts the request.
• Add regression coverage proving selected funded traffic remains zero-cost while ordinary preview-like traffic is billed normally.

Verification

N/A - backend billing/routing correctness change with no meaningful manual UI or API smoke workflow performed locally.

Visual Changes

N/A

Reviewer Notes

• MicrodollarUsageContext.is_free is the finalized per-request billing classification. Experiment attribution IDs remain independent reporting data, while accounting no longer imports or calls isFreeModel().
• The route retains the existing early rate-limit check. The quota increment moves after provider resolution so paused, unavailable, or stale-membership paid fallback requests do not consume free-model quota.