fixed model selection ui in mobile screens

[Aman071106]

PR Description

This PR fixes a RenderFlex overflow issue in the AIModelSelectorDialog that occurred on mobile-width screens (#969).

Changes Implemented:

• Updated the "Select Model Provider" section in ai_model_selector_dialog.dart to use a flexible layout.
• Wrapped ADDropdownButton in an Expanded widget with isExpanded: true to ensure it respects screen width constraints and prevents overflow.
• Improved the UI design by positioning the dropdown on the right and hiding the default arrow icon for a cleaner look, as per design feedback.

Related Issues

• Closes RenderFlex overflow while selecting model provider on mobile screens #969

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• Yes
• No, and this is why: UI fix verified manually, video/screenshots attached in issue.

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux