Downgrade non-urgent _sort_by_order logging error to warning

)

* Fix migrate_extra_fields to null m/n for non-m_of_n mastery models

Old-style exercise extra_fields were being migrated with non-null m/n
values for non-m_of_n mastery models (e.g. do_all, num_correct_in_a_row).
The mastery criteria JSON schema requires m and n to be null for these
models, causing frontend AJV validation to fail and exercises to appear
incomplete.

- Fix migrate_extra_fields to set m/n to null for non-m_of_n models
- Add management command fix_exercise_extra_fields to repair existing
  data in production

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Migrate old-style exercise extra_fields on ricecooker import

The ricecooker import path (create_node) was writing old-style
extra_fields directly to the DB without running migration. This
ensures incoming exercise data is migrated to the new-style
options.completion_criteria format before being persisted.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Cleanup management command and add more logging.

* Target dry_run more focally for better info, cleanup redundant save.

* Lazy loading, lazy coding.

* Mark_complete returns an empty list if complete.

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

…mpletion criteria fixed but their complete value is still wrong

Update snackbar to use multiline overflow option

Add link to new DCMA form

Add additional completion to exercises that have already had their completion criteria fixed but their complete value is still wrong