Delete reasoning_content in next conversation

[just-wang]

Summary

DeepSeek's official recommendation is that the thinking content should not be carried over to the next conversation's prompt.
Fixes #13692

Screenshots

Before	After

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

[crazywoola]

We actually moved this <details.*?</details> out of the backend in new version recently. We only render it in the frontend.