test: add unit tests for rubrics service

[ugoocreates-pixel]

Closes #1262

Adds comprehensive unit testing for RubricsService to ensure its behavior is verified and regressions are prevented.

Changes include:

• A new NestJS Test.createTestingModule setup using mocked repositories and a mocked TypeORM DataSource transaction runner.
• Happy-path tests for all public methods (create, findOne, findAll, update, remove).
• Coverage for edge-cases and error-handling paths such as:
  • Validation errors during creation (e.g. out-of-bounds defaultLevelIndex, insufficient maxPoints, or autoGradeEnabled requirement checks).
  • Validation errors during update (e.g. attempting to enable auto-grading when criteria are missing defaults).
  • NotFoundException when a rubric cannot be fetched.
  • ForbiddenException for unauthorized owner actions on updates and removals.

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[RUKAYAT-CODER]

Well done on the job done so far!
Kindly fix workflow to pass