feat: split read vs write authz checks for library updates

[wgu-taylor-payne]

Description

Updates the downstream sync v2 endpoints to use courses.view_library_updates for read access (list/summary) and courses.manage_library_updates for write access (sync/decline).

Previously, the list endpoint used has_studio_read_access (legacy only, no authz integration), and the sync/decline endpoints used has_studio_write_access. This meant Course Auditors were blocked from even viewing library update status. Per the design in openedx/openedx-authz#283, auditors should see the library updates page in read-only mode.

User roles impacted: Course Auditor — can now view (but not sync/decline) library updates when authz is enabled.

Changes:

• cms/djangoapps/contentstore/rest_api/v2/views/downstreams.py:
  • DownstreamListView.get: Use COURSES_VIEW_LIBRARY_UPDATES for course-filtered requests
  • DownstreamSummaryView.get: Use COURSES_VIEW_LIBRARY_UPDATES instead of COURSES_VIEW_COURSE
  • _load_accessible_block: Use COURSES_MANAGE_LIBRARY_UPDATES for write, COURSES_VIEW_LIBRARY_UPDATES for read
  • Removed unused has_studio_read_access/has_studio_write_access imports
• Tests: 5 new tests verifying editor can list+sync, auditor can list but not sync, unauthorized gets 403

Rollback: Gated behind AUTHZ_COURSE_AUTHORING_FLAG — disabling the flag triggers legacy fallback via user_has_course_permission.

Supporting information

• Closes Add courses.view_library_updates enforcement in openedx-platform openedx-authz#398
• Related: Design: Course Editor & Course Auditor screen restrictions openedx-authz#283

Testing instructions

1. Enable AUTHZ_COURSE_AUTHORING_FLAG for a course with library-linked components
2. Assign a user the course_auditor role
3. GET /api/contentstore/v2/downstreams/?course_id={course_id} → 200
4. GET /api/contentstore/v2/downstreams/{course_id}/summary → 200
5. POST /api/contentstore/v2/downstreams/{usage_key}/sync → 404 (permission denied)
6. Assign course_editor → sync should succeed (200 or 400 depending on upstream state)
7. User with no role → list returns 403

Deadline

None

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