Dify #38522: guard KB deletion during active indexing

Job description

Source issue: https://github.com/langgenius/dify/issues/38522 Repo: https://github.com/langgenius/dify Task: Fix or propose a robust implementation for preventing knowledge base deletion while documents are still indexing. The issue suggests pausing/awaiting in flight indexing before delete, or rejecting deletion with a clear error if indexing is active. Deliverables: Review the issue and the affected dataset/indexing code paths. Implement a safe fix or a well argued patch that closes the race. Include tests or a clear validation plan for the deletion/indexing scenario. Share the exact files changed and any operational caveats. Acceptance criteria: The solution directly addresses issue 38522. Deleting a knowledge base no longer races active indexing work. The proposed behavior is clear, bounded, and maintainable. Include links or commit references for the work product. Please mention issue 38522 in your application and summarize your proposed approach.