fix: let course_id bypass library access when reviewing pending updates

[efortish]

Description

The library container endpoints and the xblock embed endpoint used by the "review pending changes" modal (opened from a course unit when a library-sourced component has updates available) only checked direct library permissions. That means a Course Auditor, or anyone else holding courses.view_library_updates at the course level, gets a 403 trying to review changes even though they can view everything else about that course.

This adds an optional course_id query param to the three affected endpoints:

• GET /api/libraries/v2/containers/<container_id>/
• GET /api/libraries/v2/containers/<container_id>/children/
• GET /xblocks/v2/<usage_key>/embed/<view_name>/

When course_id is present and the requesting user holds courses.view_library_updates in that course, access is granted without requiring direct library permissions. Without it, or if the check fails, the existing library-level permission check runs exactly as before, so nothing changes for callers that don't pass it.

This is the same approach already used for the sync endpoint in #39009 and #39055.

The companion frontend change (threading the course id through to these three calls) is in openedx/frontend-app-authoring#3251.

Related to openedx/openedx-authz#441

Testing

• Added regression tests covering the course_id bypass and the "no access to an unrelated course" case for all three endpoints, plus unit tests for the new user_has_course_permission_from_query_param helper.
• Verified manually end to end against a devstack: a Course Auditor with zero library permissions gets 403 on all three endpoints without course_id, and 200 once course_id points at their course.

Deadline

None

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