fix: 17712-get-messages-api-encountered-internal-server-error

[crazywoola]

Summary

"retriever_resources": fields.Raw(
            attribute=lambda obj: json.loads(obj.message_metadata).get("retriever_resources", [])
            if obj.message_metadata
            else []
 ),

Tip

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Checklist

Important

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• 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