test: migrate dataset service document indexing tests to testcontainers

[Desel72]

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Summary

• Migrate remaining mock-based dataset service document indexing tests from
  api/tests/unit_tests/services/test_dataset_service.py (129 lines, 4 tests) into
  the existing testcontainers integration test at
  api/tests/test_containers_integration_tests/services/test_dataset_service.py
  (708 lines, 22 tests).

• The existing testcontainers file covered dataset CRUD, permission checks, and retrieval
  model updates. This PR adds 4 new tests for document pause/recover/retry orchestration
  (state transitions, DB commits, Redis flags, Celery task dispatch). External services
  (Redis, Celery) remain mocked. Then removes the fully superseded 129-line mock-based file.

Part of #32454

Screenshots

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

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