fix(core): restore visible keyboard focus ring on Collapsible trigger

[bhamodi]

Summary

Collapsible's trigger is a real <button>, but styles.trigger starts with all: unset (packages/core/src/Collapsible/Collapsible.tsx:54), which wipes the browser's default focus outline. No :focus-visible style was reintroduced anywhere in the file, so keyboard users could Tab to the trigger but saw no focus indicator -- a WCAG 2.4.7 (Focus Visible) failure.

This restores a keyboard-only focus ring on the trigger using the design system's established convention: outline: 2px solid var(--color-accent) with outlineOffset: 2px, both gated on :focus-visible. The treatment mirrors the TabMenu trigger button (packages/core/src/TabList/TabMenu.tsx:93-100), the closest structural analog (another reset trigger button that restores its ring this way), and uses the exact same token, width, style, and offset as Button, Dialog, Pagination, Tab, and Slider. Purely additive; the ring only appears on keyboard focus, so mouse/touch interaction is unchanged.

Changes

• Collapsible/Collapsible.tsx: add outline / outlineOffset :focus-visible rules to styles.trigger (restores the ring removed by all: unset).

Test plan

StyleX compiles styles to atomic class names, so jsdom cannot assert the computed outline value -- the existing :focus-visible tests in the repo (Slider, Token, FileInput) verify focus behavior by mocking matches(':focus-visible'), not the outline CSS itself. There is no established pattern for asserting a focus-ring style value, so I did not add a brittle class-name-snapshot test.

• node_modules/.bin/vitest run --root . packages/core/src/Collapsible -- 32 pass (existing suite stays green).
• node_modules/.bin/tsc --project packages/core/tsconfig.json --noEmit -- clean.
• node_modules/.bin/eslint packages/core/src/Collapsible/Collapsible.tsx -- clean.
• Manual verification: Tabbing to the trigger in Storybook should now show the accent focus ring (matching TabMenu). I could not run a browser in this environment, so this step is un-run; the values are copied verbatim from already-shipping components that render the ring correctly.

Notes

Found during a broader a11y audit of core components. Scoped to only the focus-visible style on the Collapsible trigger -- aria-controls wiring is covered separately by #3707, and the chevron/other styling is untouched.

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[cixzhang]

Thanks @bhamodi — clean :focus-visible ring, verbatim TabMenu match, semantic token, verified against StyleX CAPABILITIES. 🙏