fix(template-no-empty-headings): recognize quoted-mustache aria-hidden

`isAriaHiddenTruthy` previously only handled raw TextNode and bare
MustacheStatement attribute values. The quoted-mustache form
`aria-hidden="{{true}}"` produces a `GlimmerConcatStatement` with a
single mustache part — resolve that case by descending into the single
static-literal part, mirroring the pattern established in
template-no-aria-hidden-focusable.

Leans toward "truthy" only on literal true / empty / bare-valueless to
match the rule's doc-stated ethos of fewer false positives.

Author: johanrd

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

@@ -39,6 +39,31 @@ function isAriaHiddenTruthy(attr) {
       return value.path.value.toLowerCase() === 'true';
     }
   }
+  if (value.type === 'GlimmerConcatStatement') {
+    // Quoted-mustache form like aria-hidden="{{true}}" or aria-hidden="{{x}}".
+    // Only resolve when the concat is a single static-literal part; any
+    // dynamic path makes the runtime value unknown. Lean toward "truthy"
+    // only on literal `true` / empty-literal / bare-valueless to stay aligned
+    // with the doc-stated ethos (fewer false positives — don't flag headings
+    // the author has intentionally decorated with aria-hidden).
+    const parts = value.parts || [];
+    if (parts.length === 1) {
+      const only = parts[0];
+      if (only.type === 'GlimmerMustacheStatement' && only.path) {
+        if (only.path.type === 'GlimmerBooleanLiteral') {
+          return only.path.value === true;
+        }
+        if (only.path.type === 'GlimmerStringLiteral') {
+          return only.path.value.toLowerCase() === 'true';
+        }
+      }
+      if (only.type === 'GlimmerTextNode') {
+        const chars = only.chars.toLowerCase();
+        return chars === '' || chars === 'true';
+      }
+    }
+    return false;
+  }
   return false;
 }