fix(template-no-empty-headings): recognize boolean aria-hidden as hidden

Before: isHidden only matched aria-hidden="true" as a string literal.
Boolean / valueless / empty / mustache forms (<h1 aria-hidden />,
<h1 aria-hidden="" />, <h1 aria-hidden={{true}} />) slipped past as
"not hidden", so empty headings in those forms were flagged as empty
even when the author had intentionally hidden them from AT.

Fix: extract isAriaHiddenTruthy(). Recognize:
- valueless attribute (HBS AST has value=null or empty-string TextNode)
- "true" string literal (preserved)
- "" empty string
- {{true}} boolean mustache literal
- {{"true"}} string mustache literal

Per HTML boolean-attribute semantics (and jsx-a11y/vue-a11y convention),
presence of aria-hidden without an explicit "false" value is treated as
truthy. The strict ARIA spec treats bare aria-hidden as "undefined"
rather than "true", but every major linter in the ecosystem (and most
screen readers) treats it as true.

Four new test cases covering each of the recognized forms.

Author: johanrd

lib/rules/template-no-empty-headings.js

@@ -1,17 +1,37 @@
 const HEADINGS = new Set(['h1', 'h2', 'h3', 'h4', 'h5', 'h6']);
 
+function isAriaHiddenTruthy(attr) {
+  if (!attr) {
+    return false;
+  }
+  const value = attr.value;
+  // Valueless or empty-string attribute — <h1 aria-hidden />. Per HTML boolean
+  // attribute semantics (and jsx-a11y/vue-a11y convention), presence = truthy.
+  if (!value || (value.type === 'GlimmerTextNode' && value.chars === '')) {
+    return true;
+  }
+  if (value.type === 'GlimmerTextNode') {
+    return value.chars === 'true';
+  }
+  if (value.type === 'GlimmerMustacheStatement' && value.path) {
+    if (value.path.type === 'GlimmerBooleanLiteral') {
+      return value.path.value === true;
+    }
+    if (value.path.type === 'GlimmerStringLiteral') {
+      return value.path.value === 'true';
+    }
+  }
+  return false;
+}
+
 function isHidden(node) {
   if (!node.attributes) {
     return false;
   }
   if (node.attributes.some((a) => a.name === 'hidden')) {
     return true;
   }
-  const ariaHidden = node.attributes.find((a) => a.name === 'aria-hidden');
-  if (ariaHidden?.value?.type === 'GlimmerTextNode' && ariaHidden.value.chars === 'true') {
-    return true;
-  }
-  return false;
+  return isAriaHiddenTruthy(node.attributes.find((a) => a.name === 'aria-hidden'));
 }
 
 function isComponent(node) {

tests/lib/rules/template-no-empty-headings.js

@@ -43,6 +43,14 @@ ruleTester.run('template-no-empty-headings', rule, {
     '<template><h1><this.Heading /></h1></template>',
     '<template><h2><@heading /></h2></template>',
     '<template><h3><ns.Heading /></h3></template>',
+
+    // aria-hidden as a boolean / valueless / empty / mustache attribute — all
+    // should exempt the heading (aligns with jsx-a11y / vue-a11y treatment of
+    // boolean HTML attributes).
+    '<template><h1 aria-hidden></h1></template>',
+    '<template><h1 aria-hidden=""></h1></template>',
+    '<template><h1 aria-hidden={{true}}></h1></template>',
+    '<template><h1 aria-hidden="true">Visible to sighted only</h1></template>',
   ],
   invalid: [
     {