Story 19.4 - Add automated tests for feature parity

[acbart]

As an engineering team, we want automated tests so that the reimplementation preserves behavior.

Acceptance criteria:

• Unit tests cover:

  • assignment settings serialization
  • virtual file namespace resolution
  • file visibility/editability
  • feedback category mapping
  • queued input behavior
  • part-id extraction/replacement

• Integration tests cover:

  • run success
  • syntax error
  • runtime error
  • input program
  • instructor feedback
  • on-eval
  • on-change
  • file import/read
  • upload/download

• End-to-end tests cover learner and instructor workflows.

• Tests run in CI.