Fix incorrect exception reference for json.dumps()

[hyongtao-code]

Fixes: #24328

This PR corrects a misleading exception reference related to Python’s json module.
json.dumps() was previously described as potentially raising json.JSONDecodeError, which is not possible.

json.dumps()

• Purpose: serialize a Python object into a JSON string.
• Possible error: TypeError when the object is not JSON-serializable (e.g., contains set, bytes, or unsupported custom objects).
• It does not raise json.JSONDecodeError.

json.loads()

• Purpose: deserialize a JSON string into a Python object.
• Possible error: json.JSONDecodeError when the input string is not valid JSON.

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Summary

Screenshots

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Checklist

• 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

[hyongtao-code]

Thanks for your review.