fix: correct linewrap think display in generic openai api

[xhebox]

Summary

Follow up #13234.

I've got keys from deepseek and siliconflow. Thus I tested and removed duplicated code.

I also found two corner cases and one bug:

1. \n\n..\n may be there. That said you can't just replace \n\n but also \n in the same line. I made a regexp replace.
2. both reasoning_content and content may be empty sometimes in the thinking phase. the current code will terminate the reasoning phase, append > think_icon again.
3. delta_content is not assigned correctly in the generic openai api module.

Since official deepseek api does not respond after fixing problem 2, I don't have screen shots for it.

Screenshots

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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