fix: expose multi-value Apply button to assistive technology

[LouisDeconinck]

Fixes #1600

Summary

The multi-value ad hoc filter Apply button is rendered as a sibling of the floating dropdown. Floating UI therefore marks its wrapper as aria-hidden, and the explicit tabIndex=-1 prevents keyboard users from reaching it.

This change:

• registers the Apply button wrapper with Floating UI getInsideElements so it remains in the accessibility tree;
• restores the button native tab stop.

Validation

• corepack yarn workspace @grafana/scenes test src/variables/adhoc/AdHocFiltersVariable.test.tsx --watch=false --runInBand (362 passed)
• corepack yarn workspace @grafana/scenes typecheck
• corepack yarn workspace @grafana/scenes lint
• corepack yarn workspace @grafana/scenes build
• Prettier check on all changed files
• git diff --check

[github-actions]

Signed commits report

All 1 commit between main and fix/adhoc-apply-button-a11y-1600 have verified signatures. ✅

[cla-assistant]

All committers have signed the CLA.

[LouisDeconinck]

I've completed the Grafana CLA and updated the branch with a verified signed commit. Thanks — happy to address any review feedback.

[Sergej-Vlasov]

@LouisDeconinck thank you for the contribution! Squad will review this ASAP!
@samjewell could you give this a local test to see if it fully fixes your issue?