fix: Issues related to the deletion of conversation_id #12488

[jiandanfeng]

Summary

When the conversation_id is deleted, chat with conversation_id again, returning Conversation Not Exists.

Fixes #12488

Screenshots

Before
Can chat normally with deleted conversation_id
After

Checklist

Important

Please review the checklist below before submitting your pull request.

• 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 dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods