feat: Only return the grades tab in CCX if gradebook is enabled

[brianjbuck-wgu]

Description

The CCX Coach v2 metadata endpoint shows the student_grades tab unconditionally. CCX_COACH_TABS in lms/djangoapps/ccx/api/v2/serializers.py was a static tuple, so the tab was returned even when the gradebook it depends on is not available.

The Student Grades experience is backed by the writable gradebook. When either is missing, the Instructor Dashboard MFE still rendered the Student Grades tab that could not work.

Supporting information

Issue: #39148

Testing instructions

Manual:

1. Enable CUSTOM_COURSES_EDX, set enable_ccx: true on the course, act as the CCX Coach, and create a CCX.
2. With grades.writable_gradebook disabled, GET /api/ccx_coach/v2/courses/<ccx_id>/metadata → tabs contains enrollments, schedule, grading_policy and not student_grades.
3. Enable grades.writable_gradebook for the CCX key (ccx-v1:...) and ensure WRITABLE_GRADEBOOK_URL is set → student_grades now appears.
4. Enable the flag but unset WRITABLE_GRADEBOOK_URL → the tab is absent again.
5. Confirm the other three tabs and their sort_order values are unchanged in all cases.

Deadline

None

Other information

• No migrations

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