[TD] Align TestFunction required_file Contract

[ArthurCRodrigues]

Summary

The TestFunction ABC exposes a required_file_type property, but every concrete test class defines required_file instead. The abstract property defaults to None, so the contract is unenforced and inconsistent.

Scope

• Pick a single property name (per roadmap, required_file) and declare it on the ABC.
• Update all template tests to implement the new abstract property and remove the mismatched variant.
• Adjust metadata generation to consume the standardized property.

Impacted Files

• autograder/models/abstract/test_function.py
• All files under autograder/template_library/** containing test classes

Risks

• Missing a template override would break runtime attribute access; add tests that walk every test class and ensure the property exists.

Acceptance Criteria

• ABC enforces the property name, templates comply, and template metadata output remains correct.

References

• docs/roadmaps/TECHNICAL_DEBT_ROADMAP.md (Item 13)