[TD] Remove Stale test_library Field

[ArthurCRodrigues]

Summary

CriteriaConfig still declares an unused test_library field that conflicts with the grading config and is ignored everywhere. Config authors may think it matters, and validation silently accepts it.

Scope

• Delete the test_library attribute from CriteriaConfig.
• Remove test_library keys from all sample criteria files (e.g., docs/criteria_example.json, templates, demos).
• Ensure Pydantic config still uses extra="forbid" so stale configs fail fast.

Impacted Files

• autograder/models/config/criteria.py
• docs/criteria_example.json and any other shipped criteria configs

Risks

• Downstream configs that still include the field will now raise validation errors; communicate via release notes.

Acceptance Criteria

• CriteriaConfig loads without test_library.
• Tests covering criteria loading pass with updated fixtures.

References

• docs/roadmaps/TECHNICAL_DEBT_ROADMAP.md (Item 3)