fix: sys_var startwith 'sys.' not 'sys' #8421

[yanxiyue]

Checklist:

Important

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• Please open an issue before creating a PR or link to an existing issue
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Description

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Fixes #8421

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

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