test(Collapsible): add keyboard toggle tests for non-native activator elements

[sridhar-3009]

Summary

• CollapsibleActivator correctly handles Enter/Space in onKeydown and sets role='button' for non-native elements, but every existing keyboard test uses the default as='button' element
• The non-button activator describe block already covers role='button'/tag/no-type-attribute and a click-based toggle, but nothing verified that a non-native activator actually toggles on Enter/Space
• Adds two tests mounting with activatorAs: 'div': Enter opens, Space opens

What changed

• index.browser.test.ts: two new tests in the existing non-button activator block, matching the style already used by the open via keyboard block — a real keydown DOM event via @vue/test-utils, not a captured render-prop handler

Rebase note

Rebased onto master's post-#655 test layout: index.test.ts was consolidated into index.browser.test.ts as part of the 1.0 component-spine promotion, and the test style moved from capturing render-prop handlers to triggering real DOM events. Rewrote both new tests in that style rather than reintroducing the old pattern — the role='button' coverage I originally added is no longer needed since it already exists on master (should expose role=button and no native type when rendered as a div).

Test plan

• Enter key with activatorAs: 'div' toggles collapsible open
• Space key with activatorAs: 'div' toggles collapsible open
• All 23 Collapsible tests pass (pnpm test:run --project v0:browser)

[github-actions]

✅ Changeset found — this change will be included in the next release. Thanks!

[johnleider]

This PR currently has merge conflicts with its base branch (master) and can't be merged as-is. Could you rebase onto the latest master and resolve the conflicts when you get a chance?

Context: we're adopting a git-flow branch model — master for fixes/patches, dev for features (minor), next for breaking changes (major). This PR is correctly targeting master; it just needs a rebase to clear the conflicts. Thanks!