collab: Return `trial_started_at` alongside billing preferences (#30143)

This PR updates the `GET /billing/preferences` endpoint to return the
user's `trial_started_at` timestamp alongside the billing preferences.

Release Notes:

- N/A
582ad845b9Marshall Bowers committed on 5/7/2025, 4:49:54 PM· committed by GitHubparent1b3140d
1 file changedLine totals unavailable