chore: restricted run allowed admin autocomplete_fields

[iloveagent57]

Post-review

• Squash commits into discrete sets of changes
• Ensure that once the changes have been deployed to stage, prod is manually deployed

[Copilot]

Pull request overview

This PR improves Django admin usability for managing RestrictedRunAllowedForRestrictedCourse relationship records by enabling autocomplete widgets for the course and run foreign key fields.

Changes:

• Added autocomplete_fields for course and run in RestrictedRunAllowedForRestrictedCourseAdmin to make selecting related objects scalable and faster in the admin UI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.91%. Comparing base (0e97627) to head (ec98d53).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #133   +/-   ##
=======================================
  Coverage   85.91%   85.91%           
=======================================
  Files         109      109           
  Lines        6750     6750           
  Branches      833      833           
=======================================
  Hits         5799     5799           
  Misses        822      822           
  Partials      129      129           

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