Fix model selector scrolling and add search

[falfaddaghi]

Summary

• fix broken scrolling in the model dropdown inside the floating workflow generator panel
• add in-dropdown model search
• add keyboard navigation with arrow keys, Enter, and Escape

Details

• replace the nested Radix scroll area in the model selector with a native scroll container to prevent wheel/trackpad events from being hijacked by the parent panel
• stop wheel propagation from the dropdown list
• add a sticky search input that filters by provider/group label, display name, and raw provider/model value
• auto-focus the search field when the dropdown opens
• highlight and auto-scroll the active option during keyboard navigation

Validation

• npx eslint src/nodes/shared/model-select.tsx

[Nasrallah-AL]

This MR will be merged once nexus-bridge-proxy is integrated. The integration is required to make model selection more accurate — currently it uses a generic selection, but the correct model varies depending on the CLI tool being used.

• Model selection should reflect the active CLI tool, not a generic default
• Blocked on: nexus-bridge-proxy integration