fix: UnboundLocalError in BulkRoleCache.prefetch for authz compat roles

[efortish]

Description

Fixes [openedx-authz#514]

BulkRoleCache.prefetch() contains two loops: one processes legacy CourseAccessRole records and builds a compat_role object for each row, while the other processes Open edX AuthZ compatibility roles. However, the AuthZ loop iterates over role but mistakenly adds compat_role to the cache instead.

# openedx-authz roles
for user in users:
    compat_roles = get_authz_compat_course_access_roles_for_user(user)
    for role in compat_roles:
        course_id = get_role_cache_key_for_course(role.course_id)
        user_roles_set_for_course = roles_by_user[user.id][course_id]
        user_roles_set_for_course.add(compat_role)  # Should be `role`

This caused two distinct issues:

• UnboundLocalError: If no legacy roles were processed for any user in the batch, compat_role was never assigned, causing prefetch() to crash.
• Incorrect role caching: If the legacy loop ran, the AuthZ loop silently cached the last legacy role processed instead of the actual AuthZ role.

This bug affected the Gradebook because bulk_gradebook_view_context() calls BulkRoleCache.prefetch(). As a result, the Gradebook could fail for courses where a listed user had an AuthZ compatibility role but no legacy CourseAccessRole. Any other caller of prefetch() was susceptible to the same issue.

The fix replaces compat_role with role when adding the AuthZ role to the cache, ensuring that the correct role is stored regardless of whether legacy roles were processed.

Testing

• Added BulkRoleCachePrefetchTestCase to test_roles.py, covering two scenarios:
  • A user with only an AuthZ compatibility role, which previously triggered UnboundLocalError.
  • A batch containing legacy and AuthZ roles across different users, which previously caused the wrong role to be cached for the AuthZ user.
• Verified that both tests fail against the original implementation and pass with the fix.
• Confirmed that ruff, pylint, and lint-imports pass.
• Ran the full common/djangoapps/student test suite: 431 passed, 284 skipped.

Deadline

None.

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[rafeehcp]

We hit this on Verawood (release/verawood.1) and can confirm the fix.

We moved course roles into openedx-authz with migrate_legacy_course_roles_to_authz(..., delete_after_migration=True) and turned on authz.enable_course_authoring. Then we ran the grade report's _CourseGradeBulkContext and bulk_gradebook_view_context against that database:

• If no user in the batch has a CourseAccessRole row (the normal state once the migration deletes them), prefetch raises UnboundLocalError: cannot access local variable 'compat_role'.
• If the batch also includes one user with a legacy row (an org_course_creator_group row, which has no openedx-authz form), the staff, instructor and beta tester users get no course role in the cache.

With the one-line change in this PR, both bulk contexts cache the same roles as each user's own RoleCache. release/verawood and master still have the bug, so a backport to release/verawood would help Verawood operators who turn on course authoring.