When creating and updating chats, add a check for the parsing status of
knowledge base documents. Ensure that all documents have been parsed
before allowing chat creation to improve user experience and system
stability.

**Main Changes:**

- Add document parsing status check logic in `chat.py`.
- Implement the `is_parsed_done` method in `knowledgebase_service.py`.
- Prevent chat creation when documents are being parsed or parsing has
failed.

### What problem does this PR solve?

fix this bug：https://github.com/infiniflow/ragflow/issues/5960

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: wenju.li <wenju.li@deepctr.cn>