fix: auto-commit delete_where()

[cloudyun888]

Summary

• wrap Table.delete_where() in the same connection context manager used by delete() and update()
• make deletes visible to a new connection immediately instead of leaving them uncommitted
• add a file-backed regression test that reproduces the missing auto-commit behavior

Testing

• .venv/bin/python -m pytest tests/test_delete.py -q
• .venv/bin/python -m py_compile sqlite_utils/db.py tests/test_delete.py
• manual repro: delete rows from a file-backed database, reopen it, verify row count stays at 0

Fixes #159

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📚 Documentation preview 📚: https://sqlite-utils--731.org.readthedocs.build/en/731/