feat(new-vendor): replace Autocomplete with AutoCompleteField for assignees

[Innagl]

Describe your changes

Migrated the use cases field in Modals/NewVendor/index.tsx from the raw MUI Autocomplete component to the shared "AutoCompleteField" wrapper.

Write your issue number after "Fixes "

Fixes #3762

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• I deployed the code locally.
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• I have included the issue # in the PR.
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• The issue I am working on is assigned to me.
• I have avoided using hardcoded values to ensure scalability and maintain consistency across the application.
• I have ensured that font sizes, color choices, and other UI elements are referenced from the theme.
• My pull request is focused and addresses a single, specific feature.
• If there are UI changes, I have attached a screenshot or video to this PR.

[MuhammadKhalilzadeh]

Thank you @Innagl