fix(a11y): track inert elements as hidden in snapshots only

[agg23]

Expands on #36947 to only apply breaking changes to ARIA snapshots and not general visibility checks.

Resolves #36938.

Implement the somewhat vague inert HTML attribute spec for our accessibility tree.

My interpretation is that nodes are marked hidden, equivalent to aria-hidden, if they possess the inert attribute. This cascades to all "flat tree descendants", which are the standard descendants plus special handling for shadow DOM.

Some nodes can additionally opt out (by virtue of their tag, not by specifying a property, as it is not possible to supply an explicit false flag to inert, just the absence of the attribute). The spec lists modal dialogs as an example of a tag that escapes inert.

This opt out behavior is not supported by this PR

[github-actions]

Test results for "tests 1"

3 flaky

⚠️ [firefox-library] › library/inspector/cli-codegen-1.spec.ts:1079 › cli codegen › should not throw csp directive violation errors `@firefox-ubuntu-22.04-node18`
⚠️ [firefox-page] › page/page-emulate-media.spec.ts:144 › should keep reduced motion and color emulation after reload `@firefox-ubuntu-22.04-node18`
⚠️ [playwright-test] › ui-mode-trace.spec.ts:397 › should work behind reverse proxy `@macos-latest-node18-1`

46633 passed, 805 skipped

---

Merge workflow run.

[dgozman]

I'm not sure we should diverge getByRole() and toMatchAriaSnapshot(). I am fine to diverge snapshotForAI() though - will that work?

[agg23]

We've decided we don't want this breaking change.