Model picker search input styling and separator improvements

[daviddossett]

• Add focusFilterOnOpen option to IActionListOptions for focusing the filter input when the list opens
• Remove separator between Auto and promoted models
• Add separator above "Manage Models..." that always appears, tied to the Other section when it exists
• Add padding above list items below the filter container
• Support section on separator items so they collapse with their section
• Integrate auxiliary actions (e.g. "Manage Models...") into the main action list with proper separator and accessibility roles
• Remove filterPosition option, always place filter on top
• Remove filterInputClassName option and font-size override

[Copilot]

Pull request overview

This PR improves the model picker's search input styling and separator behavior to enhance the visual hierarchy and user experience. The changes add flexible filtering options to the ActionList component and refine how auxiliary actions (like "Manage Models...") are presented in action widget dropdowns.

Changes:

• Added filter customization options to IActionListOptions including filter position, focus behavior, and custom CSS class support
• Updated model picker to use a minimal-styled search input that inherits font size and has custom spacing
• Removed separator between Auto and promoted models; added conditional separator before "Manage Models..." that only appears when Other Models section exists
• Enhanced ActionList to support section-aware separators that collapse with their parent section
• Refactored ActionWidgetDropdown to integrate auxiliary actions into the main list with proper accessibility roles

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/vs/workbench/contrib/chat/browser/widget/input/chatModelPicker.ts	Removed separator after Auto section, changed "Manage Models..." icon to blank, added conditional separator with section support, configured filter options (top position, focus on open, minimal class)
src/vs/platform/actionWidget/browser/actionWidgetDropdown.ts	Integrated auxiliary actions into main list with separator, prevented them from updating selectedOption, changed accessibility role to 'menuitem' for non-checkable auxiliary actions
src/vs/platform/actionWidget/browser/actionWidget.css	Added minimal class styling for filter input (inherited font size, adjusted padding, controlled borders)
src/vs/platform/actionWidget/browser/actionList.ts	Added filterPosition, focusFilterOnOpen, and filterInputClassName options; implemented section-aware separator filtering; updated focus logic to optionally focus filter on open