fix(label-content-name-mismatch): ignore nested group content in visible text - #5509

label-content-name-mismatch counts every visible descendant as part of an element's visible label. For an expanded tree item, that includes the text of the child items in its nested role="group". This is the APG nested tree pattern, and the reported markup is:

<li role="treeitem" aria-label="Documents" aria-expanded="true">
  <span>Documents</span>
  <ul role="group">
    <li role="treeitem" aria-label="Report"><span>Report</span></li>
    <li role="treeitem" aria-label="Notes"><span>Notes</span></li>
  </ul>
</li>

The parent's visible text came out as Documents Report Notes, so every expanded parent item was reported as a violation now that the rule is on by default. The child items pass.

Browsers already leave nested groups out of name-from-content. Chrome names that item "Documents", and the same holds for role="group foo", role="foo group" and <fieldset>; <details> content is kept. This PR makes the rule's visible text match:

• visibleVirtual gets an opt-in excludeRoles option. A child element whose role (with fallback roles) is listed contributes no text. details is never excluded, and neither is the element itself. No other caller passes the option, so their output is unchanged.
• The check passes a new excludeRoles option, defaulting to ["group"], documented in doc/check-options.md. Setting it to [] restores the old behavior.

Notes for reviewers:

• The rule is already limited to widgets that are named from content, and so is ACT 2ee8b8. treeitem is one of those widgets, so the fix belongs in how visible text is gathered, not in what the rule applies to.
• The exclusion applies to whatever element is being checked, not only tree items. For example, a role="group" inside a button is also skipped. This matches Chrome, which also leaves group content out of a link's name from content.
• Chrome skips other nested container roles too (menu, listbox, tree, grid, toolbar, landmarks, …). This PR deliberately covers only group, which is what the issue reports.
• This touches the same recursion in visibleVirtual as fix(commons/text/visibleVirtual): separate the text of adjacent block elements #5460 and fix(commons/text/visibleVirtual): separate the words around a visually hidden element #5476, and adds an integration fixture #pass20, as fix(label-content-name-mismatch): match labels inside parenthesised name text #5499 does. Whichever merges later will need a small rebase and renumbering.

Verification:

• New unit tests (visibleVirtual and the check) fail without the change and pass with it. The guard tests (button in a gridcell, nested list, unnamed region still counted) were checked by widening the role list.
• New integration fixture #pass20 is reported as a violation with excludeRoles: [] and passes with the default.
• #fail11, a leaf tree item whose text isn't in its name, still fails. It also fails if treeitem stops being named from content, which guards against fixing this through rule applicability instead.
• The reporter's exact markup gives three passes and no violations in Chrome 156. A real leaf mismatch is still reported.
• eslint and prettier --check pass. test:tsc, test:unit:core/commons/rule-matches/checks/api/virtual-rules/gather-internals and test:apg pass locally.
• test:unit:integration passes apart from identical-links-same-purpose #pass15, and test:act passes apart from 78fd32 (line height). Both fail the same way on develop locally and are unrelated to this change.

Closes: #5505