Modal: use inert instead of aria-hidden for non-modal isolation

[ramonjd]

What? How?

This is a bit of a YOLO PR.

Context: this was flagged during an accessibility review of the media editor modal, but the modalize() helper is shared by every Modal in Gutenberg, so the change is split out here for broader review.

Switches the Modal component's background isolation from aria-hidden to the inert attribute.

Why

aria-hidden removes elements from the accessibility tree but leaves them keyboard-accessible — users can still Tab into content behind the modal. inert does both: hidden from assistive technology and unreachable via keyboard navigation.

The inert attribute has been baseline across all browsers since 2023.

Test plan

1. Open any modal in the editor (e.g. media editor, preferences, keyboard shortcuts)
2. With the modal open, try Tab/Shift+Tab. Focus should not reach content behind the modal
3. Close the modal. Background content should be fully interactive again
4. Open a nested modal (e.g. media editor > keyboard shortcuts). Outer modal becomes inert, inner modal close restores it
5. Test with VoiceOver/NVDA. Background content should not be announced while modal is open

Before	After

[github-actions]

Size Change: -8 B (0%)

Total Size: 7.67 MB

📦 View Changed

Filename	Size	Change
build/scripts/components/index.min.js	270 kB	-8 B (0%)

_{compressed-size-action}

[ramonjd]

I think some of the e2e fails are related. not sure. i won't have much time to look at this PR before 7.1.