fix(core): link Banner expand toggle to its content via aria-controls

[bhamodi]

Summary

Banner's expand/collapse toggle exposed aria-expanded but had no aria-controls, and its collapsible content region had no id -- so the disclosure relationship (toggle button ↔ the region it shows/hides) was implicit only. Screen-reader users heard the expanded/collapsed state with no way to associate it with the region being controlled.

This adds a useId()-generated id on the content region and wires aria-controls to it on the toggle button. Because the region is conditionally rendered (mounted only while expanded), aria-controls is set only while the region is mounted, so it never points at a non-existent id. This matches the aria-controls={... ? ... : undefined} pattern already used by the Date inputs (DateInput, DateRangeInput, DateTimeInput) for their conditionally-mounted popovers. Purely additive; no behavior/visual change. Mirrors the sibling fix on Collapsible (#3707).

Changes

• Banner/Banner.tsx: useId() → id on the content region + aria-controls on the toggle button (set only while the region is mounted).

Test plan

• node_modules/.bin/vitest run --root . packages/core/src/Banner -- 33 pass, including two new tests (both confirmed failing before the fix):
  • links the expand toggle to its content region via aria-controls -- asserts the toggle's aria-controls resolves via document.getElementById to the region containing the banner children.
  • sets aria-controls only while the content region is mounted -- asserts there is no aria-controls while collapsed (region unmounted, no dangling reference), and that it resolves to the mounted region after expanding.
• node_modules/.bin/tsc --project packages/core/tsconfig.json --noEmit -- clean.
• node_modules/.bin/eslint on changed files -- clean.

Notes

Found during a broader a11y audit of core components. Scoped to one fix per PR. Button forwards arbitrary aria-* props to the underlying <button> (the same spread the existing aria-expanded relies on), so aria-controls reaches the DOM. Skipped role="region"/aria-labelledby to keep the change minimal and consistent with #3707.

[vercel]

@bhamodi is attempting to deploy a commit to the Meta Open Source Team on Vercel.

A member of the Team first needs to authorize it.