collab: Improve `GET /billing/usage` endpoint (#29898)

This PR improves the `GET /billing/usage` endpoint.

We now return the usage with the default plan limits when there is no
usage record.

Release Notes:

- N/A
45fe158bc9Marshall Bowers committed on 5/5/2025, 2:31:02 AM· committed by GitHubparent55eb071
2 files changedLine totals unavailable