Update the model modal:position the scrollbar further inside the modal

[maoguy]

fixed the Conflict between the scrollbar and the rounded corners on he right side of the "Model Confighuration Form Modal"

link for the issue👉Fix #17671

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• This change requires a documentation update, included: Dify Document
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• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
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