fix(vscode): improve screen reader model navigation

[marius-kilocode]

PR #10688 established the initial combobox and listbox semantics for the VS Code model picker. Follow-up accessibility testing by a blind user found that two essential workflows were still blocked:

• Entering a provider initial, such as N for NVIDIA, updated the rendered results but did not move virtual screen-reader focus. The screen reader continued announcing the previously active option instead of a matching result.
• Provider headings were pointer-operated controls outside the listbox navigation sequence. They could not be reached and collapsed with standard arrow-key navigation, so reaching the next provider could require traversing hundreds of models.

A grouped listbox cannot coherently represent provider headings that are both navigable parents and expandable controls. This change uses the ARIA editable-combobox-with-tree pattern instead. Provider headings are tree items with announced expanded or collapsed state, while models and the clear or default entry are child tree items. Navigation is derived from the currently visible tree, so aria-activedescendant always references a rendered node after filtering, collapsing, favorite changes, or provider updates.

The keyboard behavior follows tree conventions: Up and Down traverse visible items, Left moves to a parent or collapses it, Right expands a provider or enters its first model, Home and End move to the visible edges, Enter toggles a provider or commits a model, and Escape cancels without changing the selection. Filtering moves virtual focus to a visible matching model. Native text editing remains intact until tree navigation begins, so Left, Right, Home, End, Backspace, and Delete continue to work normally in the editable search field.

Favorites, duplicate favorite rows, clear or default selection, expanded preview behavior, pointer selection, and /model focus restoration remain predictable under the same interaction model. No Agent Manager-specific UI components are changed.

Follow-up to #10688.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (5 files)

• .changeset/fix-model-picker-navigation.md
• packages/kilo-vscode/tests/model-selector-accessibility.spec.ts
• packages/kilo-vscode/webview-ui/src/components/shared/ModelSelector.tsx
• packages/kilo-vscode/webview-ui/src/stories/shared.stories.tsx
• packages/kilo-vscode/webview-ui/src/styles/model-selector.css

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_{Review guidance: REVIEW.md from base branch main}