Fix: Handle expired OpenEdX refresh tokens gracefully

[sentry]

This PR addresses the HTTPError: 400 Client Error: Bad Request for url: https://courses.learn.mit.edu/oauth2/access_token occurring in the openedx.tasks.update_edx_user_profile Celery task.

Root Cause:
The error occurs when a user's stored OpenEdX OAuth2 refresh token has expired or been invalidated on the OpenEdX side. When _refresh_edx_api_auth attempts to use this token, the OpenEdX server returns a 400 Bad Request, which was previously unhandled, causing the task to crash.

Solution:

1. Graceful Refresh Token Handling: In openedx/api.py, the _refresh_edx_api_auth function now catches HTTPError with a 400 status code. If a 400 is received, it indicates an invalid refresh token.
2. Full Re-authorization Fallback: Upon catching a 400, the stale refresh_token and access_token are cleared from the OpenEdxApiAuth record. The system then attempts a full re-authorization by calling create_edx_auth_token, which simulates a user login to obtain a fresh set of tokens.
3. Improved Logging: A warning log is added when a refresh token fails with a 400, providing visibility into when the re-authorization fallback is triggered.
4. Robust Celery Task: The update_edx_user_profile Celery task in openedx/tasks.py now includes a try-except block. This ensures that if any part of the profile update or token re-authorization process fails (even the fallback), the task logs the exception and completes gracefully, preventing repeated crashes for the same user.

Fixes MITXONLINE-6EG

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