fix: update unit test for non-primary VAR_ID validation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: abnegate

tests/unit/Validator/StructureTest.php

@@ -773,7 +773,7 @@ public function testId(): void
             'id' => $sqlId,
         ])));
 
-        $this->assertEquals(false, $validator->isValid(new Document([
+        $this->assertEquals(true, $validator->isValid(new Document([
             '$collection' => ID::custom('posts'),
             'title' => 'My Title',
             'description' => null,