Fix ordering for mitxonline programs

[mbertrand]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/10765

Description (What does it do?)

Refactors assignment of child resource position numbers, based on the mitxonline req_tree attribute

How can this be tested?

• Run ./manage.py backpopulate_mitxonline_data
• Go to http://open.odl.local:8062/search?q=Universal+AI
• Click on the "Universal AI" program to open the drawer
• The courses should be listed in the same order as they appear here ("Fundamentals of ..." first, followed by "AI and/for ..." courses) - https://learn.mit.edu/programs/program-v1:UAI+B2C

[github-actions]

OpenAPI Changes

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[Copilot]

Pull request overview

Refactors MITx Online program child ordering so program/course relationships preserve the traversal order defined by the MITx Online req_tree, including interleaving display_mode="course" sub-programs among courses.

Changes:

• Add get_child_positions to derive stable (type, id) -> position mappings from req_tree traversal.
• Preserve requested course ID ordering in _fetch_courses_by_ids, and stamp explicit position fields onto transformed courses and child programs.
• Update program loaders to honor explicit position for course and child-program relationships, with new/updated tests covering ordering and gap behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
learning_resources/etl/mitxonline.py	Computes child positions from req_tree, preserves course fetch order, and stamps positions onto transformed program children.
learning_resources/etl/mitxonline_test.py	Adds unit tests for get_child_positions and ordered _fetch_courses_by_ids; updates program transform expectations.
learning_resources/etl/loaders.py	Honors explicit child positions when creating relationships; adjusts pass-2 child-program relationship positioning logic.
learning_resources/etl/loaders_test.py	Adds loader tests ensuring explicit course positions are respected and mixed course/sub-program ordering is preserved.

[mbertrand]

I believe this comment is incorrect because of the two-pass design:

In pass 1, load_program calls learning_resource.children.all().delete(), which wipes out all prior children — both PROGRAM_COURSES and PROGRAM_PROGRAMS — and then bulk-creates fresh PROGRAM_COURSES with assigned positions.

Pass 2 only runs after pass 1 has committed. So when it computes existing_max = parent_resource.children.aggregate(Max("position")), the only children present are the freshly-created PROGRAM_COURSES from this run. Stale PROGRAM_PROGRAMS from prior runs are already gone.