feat: persist subsection prerequisites in course export/import

[Agrendalath]

Description

Subsection prerequisites (Settings -> Advanced -> "Enable Subsection Prerequisites") are stored by the gating API as milestones, not in XBlock fields. As a result, they are lost when a course is exported and imported into a different instance, and course authors have to configure them again.

This PR carries the prerequisite settings through the OLX export and applies them back through the gating API on import. It supersedes #37475 and follows the conclusions of its review:

• No new XBlock fields and no new child elements. The gating API still owns the settings.
• The data is stored as attributes of <sequential>, like the other subsection settings.
• The gating API is called directly, not through the XBlock service, which is not available in the export runtime.
• The existing validation of the gating API is reused.
• Malformed prerequisites fail the import with a clear message.
• Requirements on subsections that are not in the archive are skipped with a warning. The dangling references are removed (the gating API deactivates them, so no data is lost).

OLX format

<sequential display_name="Prerequisite" is_prereq="true">...</sequential>
<sequential display_name="Gated" prereq="df2059964cbf47c5bbd6052855a5bb87"
            prereq_min_score="80" prereq_min_completion="90">...</sequential>

• is_prereq: the subsection can be used as a prerequisite.
• prereq: the url_name (block ID) of the required subsection, the same way child pointers reference blocks.
• prereq_min_score and prereq_min_completion: omitted when empty.

The attributes are only used during import. They are never persisted in xml_attributes, so a later export cannot emit stale values.

Import behavior

The gating of the destination course is reconciled with the archive:

• Subsections that are not part of the archive lose their gating. This also removes requirements pointing to them, so replacing a course with another leaves no orphaned milestones.
• Subsections in the archive get the settings from their attributes. If the archive has no prerequisite attributes at all (meaning it predates this change), it keeps its existing settings, so restoring an old backup doesn't silently drop them.
• An unknown or invalid prerequisite fails the import with a message naming the subsection, for example: Invalid prerequisite settings of the subsection "Gated" (...): "..." is not a subsection of this course.
• Relationships that do not change are left untouched, so learners keep their fulfilled prerequisites.
• Entrance exam and prerequisite course milestones are not affected.

Supporting information

• Issue: Unit Prerequisites Lost During Course Export/Import #36995
• Forum discussion: https://discuss.openedx.org/t/request-for-comments-course-export-import-and-re-run/16508
• Previous attempt and review: feat: Configured subsection prerequisites persist during course olx/xml export/import #37475
• Docs update: Documentation updated to reflect changes in PR 37475 docs.openedx.org#1324 (merged slightly too early)

Testing instructions

1. In Studio, enable subsection prerequisites in the course's advanced settings.
2. Mark subsection A as a prerequisite. Make subsection B require A with a minimum score and completion.
3. Export the course and check sequential/<A>.xml and sequential/<B>.xml for the attributes above.
4. Import the archive into a new course, enable subsection prerequisites there, and check that B requires A with the same values.
5. Import an archive of a different course over the course from step 2. Check that no prerequisites remain.
6. Remove the prerequisite attributes from the archive from step 3 and import it over the course from step 2. Check that the prerequisites are kept.

Automated tests:

pytest xmodule/tests/test_sequence.py xmodule/modulestore/tests/test_xml_importer.py \
  cms/djangoapps/contentstore/views/tests/test_import_export.py::TestCourseExportImportSubsectionPrerequisites
pytest --ds=lms.envs.test openedx/core/lib/gating/tests/test_api.py

Deadline

None.

Private-ref: BB-11256

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[bradenmacdonald]

@ormsbee this might be of interest to you