feat: add generate exception certificates modal

[wgu-jesse-stewart]

Description

Adds comprehensive test coverage for the certificate regeneration API's student set filtering contract between the API layer and task layer.

This PR introduces RegenerateCertificatesViewTest, a new test class with three tests that verify the exact calling contract between RegenerateCertificatesView (API v2) and task_api.generate_certificates_for_students. These tests prevent silent failures if either layer is renamed independently.

User Impact: None. This is a test-only change with no functional modifications.

Context: During code review, a naming asymmetry was flagged between how different student_set values are handled:

• 'allowlisted' → translated to 'all_allowlisted' (legacy compatibility)
• 'allowlisted_not_generated' → passed through unchanged (modern native value)
• 'all' → omitted entirely (default behavior)

While the existing code is correct, there were no tests pinning this contract. These tests serve as both documentation and regression prevention.

Changes

Added RegenerateCertificatesViewTest to lms/djangoapps/instructor/tests/test_api_v2.py with three test methods:

1. test_allowlisted_not_generated_passes_correct_student_set
   Verifies that student_set='allowlisted_not_generated' is passed to the task layer without translation.

2. test_allowlisted_translates_to_all_allowlisted
   Verifies that student_set='allowlisted' is translated to 'all_allowlisted' for backward compatibility with the pre-allowlist "whitelist" naming era.

3. test_all_students_omits_student_set_kwarg
   Verifies that student_set='all' results in calling the task layer with no student_set kwarg, preserving the default "generate for all enrolled students" behavior.

Supporting information

• Jira: [Link to ticket if applicable]
• Context: This addresses defensive testing feedback from code review
• Related: Part of the instructor API v2 certificate management feature set

Testing instructions

Run the new test class:

pytest lms/djangoapps/instructor/tests/test_api_v2.py::RegenerateCertificatesViewTest -v

All three tests should pass and verify:

1. Mock assertions on task_api.generate_certificates_for_students
2. Correct positional and keyword arguments for each student set variant
3. No regressions in the API ↔ task layer contract

Deadline

None

Other information

Naming asymmetry explanation

The intentional asymmetry in student_set handling exists because:

• 'all_allowlisted' is a legacy value from the pre-allowlist "whitelist" naming era. The API layer accepts the modern term 'allowlisted' and translates it for backward compatibility with the task layer.
• 'allowlisted_not_generated' is a native value that the task layer was updated to accept directly without translation.

This asymmetry is now documented and locked in by tests, preventing future drift between the API and task layers.

Dependencies

• No external dependencies
• No database migrations
• No configuration changes required
• Safe to merge independently

Code archaeology notes

For future maintainers: If you're renaming student_set values, these tests will fail loudly rather than allowing silent misbehavior. Update both the view logic AND the tests together, ensuring the contract remains explicit.

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