docs+test: tighten allowundefined scope, add HBS test parity (Copilot review)

Author: johanrd

lib/rules/template-no-invalid-aria-attributes.js

@@ -11,12 +11,14 @@ function isNumeric(value) {
   return !Number.isNaN(Number(value));
 }
 
-// Per aria-query's `allowundefined` convention: some attribute definitions
-// (notably the 4 boolean-type attrs aria-expanded / aria-hidden / aria-grabbed
-// / aria-selected) mark the literal string 'undefined' as a spec-valid value
-// meaning "state is not applicable" (per WAI-ARIA 1.2 value table, e.g.
-// https://www.w3.org/TR/wai-aria-1.2/#aria-expanded). Any attribute type can
-// in principle accept 'undefined' via this flag.
+// In aria-query 5.3.2, `allowundefined: true` is set only on the four
+// boolean-like ARIA state attributes — `aria-expanded`, `aria-hidden`,
+// `aria-grabbed`, `aria-selected` — whose WAI-ARIA 1.2 value tables list
+// the literal string `"undefined"` as a spec-valid value meaning "state
+// is not applicable" (e.g. https://www.w3.org/TR/wai-aria-1.2/#aria-expanded).
+// The flag is nominally type-agnostic, but in practice this function only
+// green-lights `"undefined"` for that boolean-like subset; no non-boolean
+// ARIA attribute in aria-query currently sets `allowundefined`.
 function allowsUndefinedLiteral(attrDef, value) {
   return value === 'undefined' && Boolean(attrDef.allowundefined);
 }

tests/lib/rules/template-no-invalid-aria-attributes.js

@@ -190,6 +190,7 @@ hbsRuleTester.run('template-no-invalid-aria-attributes', rule, {
 
     // aria-pressed is tristate WITHOUT allowundefined; valid values:
     '<button aria-pressed="true">Toggle</button>',
+    '<button aria-pressed="false">Toggle</button>',
     '<button aria-pressed="mixed">Toggle</button>',
 
     '<button aria-label={{if @isNew (t "actions.add") (t "actions.edit")}}></button>',