fix(amazonq): add custom modal dropdown for model selection

[aseemxs]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Description

The model selection dropdown in Amazon Q chat was getting cut off at the bottom, showing only 2-3 out of 4 available models. This was caused by overflow clipping in
the JCEF webview environment.

Solution:
Implemented a custom dropdown that replaces native elements with direct fixed positioning. This approach avoids the modal overlay used in #6120 for better performance and simpler code

Key changes:

• Custom dropdown with position: fixed and high z-index to escape overflow clipping
• Smart positioning (opens above or below trigger based on available space)
• Consistent 11px font size for trigger and dropdown options
• Reduced padding for balanced visual appearance
• Generic fallback (empty string) instead of hardcoded "Select model" text
• Applies to all select.mynah-form-input elements, not just model dropdown

Supersedes #6120 - removed modal overlay approach in favor of direct positioning.

Checklist

• My code follows the code style of this project
• I have added tests to cover my changes (UI-only change in Browser.kt webview)
• A short description of the change has been added to the [CHANGELOG](https://github.com/aws/aws-toolkit-jetbrains/blob/master/CONTRIBUTING.md#contributing-via-pull
  -requests) if the change is customer-facing in the IDE.
• I have added metrics for my changes (if required) (No metrics needed for UI fix)

License

I confirm that my contribution is made under the terms of the Apache 2.0 license.