fix(template-no-empty-headings): treat aria-hidden values case-insensitively

HTML attribute value comparison is ASCII case-insensitive per spec, so
`aria-hidden="TRUE"` and `aria-hidden="True"` (and their mustache-string
equivalents) should be recognised as truthy. Mirrors the same case-
handling choice made in #2718 for `kind="captions"`.

Tests cover `"TRUE"`, `"True"`, `{{"TRUE"}}`, `{{"True"}}`.

Author: johanrd

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

@@ -10,15 +10,17 @@ function isAriaHiddenTruthy(attr) {
   if (!value || (value.type === 'GlimmerTextNode' && value.chars === '')) {
     return true;
   }
+  // HTML attribute values compare ASCII-case-insensitively, so "TRUE"/"True"
+  // count as truthy.
   if (value.type === 'GlimmerTextNode') {
-    return value.chars === 'true';
+    return value.chars.toLowerCase() === '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 value.path.value.toLowerCase() === 'true';
     }
   }
   return false;

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

@@ -51,6 +51,13 @@ ruleTester.run('template-no-empty-headings', rule, {
     '<template><h1 aria-hidden=""></h1></template>',
     '<template><h1 aria-hidden={{true}}></h1></template>',
     '<template><h1 aria-hidden="true">Visible to sighted only</h1></template>',
+
+    // HTML attribute values are ASCII case-insensitive — "TRUE" / "True" /
+    // {{"TRUE"}} / {{"True"}} all count as truthy.
+    '<template><h1 aria-hidden="TRUE"></h1></template>',
+    '<template><h1 aria-hidden="True"></h1></template>',
+    '<template><h1 aria-hidden={{"TRUE"}}></h1></template>',
+    '<template><h1 aria-hidden={{"True"}}></h1></template>',
   ],
   invalid: [
     {