fix+docs+test: trim whitespace, tighten JSDoc, add whitespace-only coverage (Copilot review)

- JSDoc for hasNonEmptyTextAttr() rewritten: no longer overstates the
  guarantee for dynamic values, and notes that aria-labelledby IDREFs
  are not validated.
- Added invalid-case coverage for whitespace-only aria-label /
  aria-labelledby / title — ACCNAME 1.2 §4.3.2 step 2D.
- hasNonEmptyTextAttr() already trims static values, so the new
  whitespace-only cases flag without further rule changes.

Author: johanrd

lib/rules/template-require-valid-alt-text.js

@@ -12,10 +12,16 @@ function hasAnyAttr(node, names) {
   return names.some((name) => hasAttr(node, name));
 }
 
-// For accessible-name fallback attributes (aria-label, aria-labelledby, title),
-// an empty string provides no accessible name — it must be checked as "any truthy
-// static value" or "any dynamic value". Returns true iff the attribute is
-// present AND will meaningfully contribute an accessible name.
+/**
+ * Returns true if the named attribute is present with a non-empty, non-whitespace
+ * static value, OR present with a dynamic (mustache/concat) value. Dynamic values
+ * are assumed to resolve to a meaningful name at runtime (we can't verify at lint
+ * time). Static empty-string / whitespace-only values return false — per ACCNAME 1.2
+ * §4.3.2 step 2D.
+ *
+ * NOTE: This does not validate that aria-labelledby IDREFs reference existing IDs.
+ * Rule consumers should layer that check separately if needed.
+ */
 function hasNonEmptyTextAttr(node, name) {
   const attr = findAttr(node, name);
   if (!attr?.value) {

tests/lib/rules/template-require-valid-alt-text.js

@@ -106,6 +106,22 @@ ruleTester.run('template-require-valid-alt-text', rule, {
       output: null,
       errors: [{ messageId: 'areaMissing' }],
     },
+    // Whitespace-only values are not valid accessible names per ACCNAME 1.2 §4.3.2 step 2D.
+    {
+      code: '<template><input type="image" aria-label=" " /></template>',
+      output: null,
+      errors: [{ messageId: 'inputImage' }],
+    },
+    {
+      code: '<template><object aria-labelledby="\n\t" ></object></template>',
+      output: null,
+      errors: [{ messageId: 'objectMissing' }],
+    },
+    {
+      code: '<template><area href="/" title=" " /></template>',
+      output: null,
+      errors: [{ messageId: 'areaMissing' }],
+    },
     {
       code: '<template><img src="/test.jpg" /></template>',
       output: null,