fix bug:support streaming mode content start with "data:"

[jifei]

Summary

When deploying LLM with ms-swift, the streaming data returned does not consistently start with "data: ". Specifically, the data does not include a leading space after "data:". This inconsistency stems from the pull request #11272 return failed，the version of Dify before 0.13.0 is ok. To address this, my code has been updated to support both formats: "data:" and "data: "

streaming content e.g.
"data:{"model": "my-llm", "choices": [{"index": 0, "delta": {"role": "assistant", "content": "Hello! How ", "tool_calls": null}, "finish_reason": null, "logprobs": null}], "usage": {"prompt_tokens": 30, "completion_tokens": 7, "total_tokens": 37}, "id": "chatcmpl-d802c95d766a41a5bf9491aafdba2b36", "object": "chat.completion.chunk", "created": 1735303124}"

Fixes #12143

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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