feat(api): enhance seat endpoint to support credit seat metadata

[rjv31]

Ticket - https://2u-internal.atlassian.net/browse/PROD-4431

This PR enhances the existing Discovery API endpoint to support credit seat metadata as part of the eCommerce deprecation effort. Specifically, we now allow credit course runs to include:

• Credit Provider (e.g., ASU, CO)
• Credit Hours
• Upgrade Deadline Override

Changes Introduced:

• Updated CourseRun seat creation logic (update_or_create_seats, update_or_create_seat_helper) to accept and persist credit metadata.
• Extended serializers to expose credit_provider, credit_hours, and upgrade_deadline_override fields for credit seats.
• Added validation to ensure data integrity.

[ankit-sonata]

@rjv31 I noticed the build is failing due to a failing test case, could you take a look at the test logs and fix the issue so that the build passes?