[FocusTrap] Guard focus restoration against non-focusable relatedTarget

[Dextheking1]

Closes #49159.

Problem

When a FocusTrap unmounts (or open flips), the cleanup restores focus to
nodeToRestore.current, which is captured from event.relatedTarget:

(nodeToRestore.current as HTMLElement).focus();

relatedTarget is typed as EventTarget | null, but nothing guarantees it
exposes a focus method. Since jsdom 30.1.0 it can resolve to a truthy value
without one (see #49159 for the full analysis), so unmounting a trap throws:

TypeError: (nodeToRestore.current as HTMLElement).focus is not a function

This previously had a defensive guard, removed in #48368 when the restore
logic was reworked.

Fix

Restore the guard, in typeof form, before calling .focus():

if (typeof (nodeToRestore.current as HTMLElement).focus === 'function') {
  ignoreNextEnforceFocus.current = true;
  (nodeToRestore.current as HTMLElement).focus();
}
nodeToRestore.current = null;

The ref is still cleared unconditionally so a subsequent focus event
re-captures a fresh target. No behavior change in real browsers, where the
target always has .focus.

Test

Added a regression test that dispatches a focusin whose relatedTarget is
truthy but has no focus method, then unmounts the trap and asserts nothing
is thrown. Verified it fails without the fix (TypeError) and passes with it.

• pnpm test:unit FocusTrap passes

[code-infra-dashboard]

Deploy preview

https://deploy-preview-49164--material-ui.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	🔺+38B(+0.01%)	🔺+10B(+0.01%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

---

Check out the code infra dashboard for more information about this PR.