feat: add content suggestions

[Henrrypg]

Summary

• Add ContentSuggestionsOrchestrator: reviews a course's full outline via OpenEdXProcessor and asks an LLM to propose per-unit content improvement suggestions (wording/structure/pedagogy/accessibility), each carrying full section/subsection/unit ancestry and an optional proposed_change (display_name/content_html/summary diff).
• Add CrossSlotSessionOrchestrator (new ScopedSessionOrchestratorchestrator.py): lets one session be shared across every UI slot (AIWorkflowScope) pointing at the same profile/course, so an author sees the same in-progress suggestions whether they open the tool from the course outline sidebar or a unit-level widget. Left ScopedSessionOrchestrator/FlashCardsOrchestrator untouched since flashcards intentionally keep separate sessions per widget.
• Suggestions are always generated and stored course-wide, then filtered per request by location_id: no location (or a location outside the course tree, e.g. the course outline) returns everything; a specific section/subsection/unit returns only its own suggestions — including correctly returning an empty list for a real unit with zero suggestions (previously fell back to showing everything).
• clear_session keeps the session row and its extra_instructions guidelines alive, clearing only the generated suggestions/known_location_ids, so re-opening the tool prefills the last-used guidelines instead of starting blank.
• Suggestions are returned course-ordered (course_suggestions) to support cross-unit "next/previous" navigation on the frontend.

The frontend for this new workflow isn't ready yet but we have a draft version here.

How it should look like:

Screencast.from.03-07-26.11.55.09.webm

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.42%. Comparing base (4c8a373) to head (d14cc27).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   95.36%   95.42%   +0.05%     
==========================================
  Files          69       71       +2     
  Lines        8088     8516     +428     
  Branches      430      451      +21     
==========================================
+ Hits         7713     8126     +413     
- Misses        281      292      +11     
- Partials       94       98       +4     

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[felipemontoya]

Thanks for the corrections. Now this is properly working.

While the UI at eduNEXT#7 is moderatly usable I would suggest we keep it as a PR in WIP mode.

We still need to create the ability to apply the fixes and a whole set of UI tools for it.