collab: Require product code for `POST /billing/subscriptions` (#30968)
This PR makes the `product` field required in the request body for `POST
/billing/subscriptions`.
We were already passing this everywhere, in practice.
Release Notes:
- N/A
b93c67438cMarshall Bowers committed on 5/19/2025, 6:11:33 PM· committed by GitHubparentfdec966