fix(aria-prohibited-attr): allow aria-label/labelledby on form

[chutchins25]

Extends the #5185 fix to the form element, which has the same false positive.

Stacked PR — targets chut/3410-body-label-prohibit (#3410), which is itself stacked on #5185. GitHub will auto-retarget as the parents merge.

What & why

Follow-up to #5244 / #5185. An unnamed <form> resolves to a null role in axe (implicit-html-roles.js: hasAccessibleName(vNode) ? 'form' : null), so aria-prohibited-attr wrongly prohibited aria-label/aria-labelledby on <form aria-labelledby="…"> with an empty/unresolved reference.

Per ARIA in HTML (#el-form): the form element allows Global aria-* attributes, and "A form is not exposed as a landmark region unless it has been provided an accessible name."

Audit result

I audited implicit-html-roles.js for the name-dependent-null-role pattern. Only section, aside (fixed in #5185) and form match. header/footer map to null based on ancestor (not accessible name), and a scoped <footer>/<header> is exposed as generic — which prohibits naming — so flagging those is correct and they are intentionally excluded.

Fix

Add form to the elementsAllowedAriaLabel option (named forms already pass via the form landmark role).

Tests

• aria-prohibited-attr check unit — form with empty aria-labelledby; named form.
• aria-prohibited-attr integration — form with empty aria-labelledby pass case.
• doc/check-options.md default updated.

Closes #5244

[chutchins25]

Superseded by #5259. Its general mechanism already allows naming on an unnamed form (the role resolves to the implicit role and form isn't namingProhibited), so the dedicated form addition here isn't needed. Closing — #5259 covers the #5244 case.