fix(dataset): clean multimodal attachments during reindex - #41999

Summary

• delete the replaced document segment attachment bindings during multimodal re-indexing
• remove attachment vectors and UploadFile rows only after confirming that no bindings still reference them, preserving shared attachments
• delete storage objects after the database commit with per-key error handling

This follows the attachment lifecycle and orphan-check behavior established by #41400 while keeping this bug fix scoped to the re-index path.

Fixes #41994

Screenshots

Not applicable. This is a backend cleanup task fix.

Testing

• api/.venv/bin/pytest -q api/tests/unit_tests/tasks/test_document_indexing_update_task.py (16 passed)
• api/.venv/bin/pytest -q api/tests/unit_tests/tasks/test_document_indexing_update_task.py api/tests/unit_tests/tasks/test_segment_index_cleanup_tasks.py api/tests/unit_tests/tasks/test_batch_clean_document_task.py api/tests/unit_tests/tasks/test_clean_document_task.py (28 passed)
• api/.venv/bin/ruff format --check api/tasks/document_indexing_update_task.py api/tests/unit_tests/tasks/test_document_indexing_update_task.py
• api/.venv/bin/ruff check api/tasks/document_indexing_update_task.py api/tests/unit_tests/tasks/test_document_indexing_update_task.py
• api/.venv/bin/pyrefly check api/tasks/document_indexing_update_task.py

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 does not apply to typos!)
• I added regression coverage for orphaned and shared attachments, plus storage deletion failures.
• I updated the documentation accordingly. (No documentation change is required.)
• I ran make lint && make type-check (backend). Targeted Ruff and Pyrefly checks passed; full Mypy is blocked locally by missing optional provider packages.

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