Add extensive controller and model tests

tests/Unit/AttachmentGroupModelTest.php

@@ -0,0 +1,33 @@
+<?php
+
+use App\Models\AttachmentExtension;
+use App\Models\AttachmentGroup;
+
+it('exposes attachment group hierarchy and extensions', function (): void {
+    $parent = AttachmentGroup::create([
+        'name' => 'Parent',
+        'max_size_kb' => 100,
+        'is_active' => true,
+    ]);
+
+    $child = AttachmentGroup::create([
+        'name' => 'Child',
+        'parent_id' => $parent->id,
+        'position' => 1,
+        'max_size_kb' => 100,
+        'is_active' => true,
+    ]);
+
+    $extension = AttachmentExtension::create([
+        'extension' => 'png',
+        'attachment_group_id' => $parent->id,
+        'allowed_mimes' => ['image/png'],
+    ]);
+
+    $parent->load(['children', 'extensions']);
+    $child->load('parent');
+
+    expect($parent->children->first()->id)->toBe($child->id);
+    expect($parent->extensions->first()->id)->toBe($extension->id);
+    expect($child->parent?->id)->toBe($parent->id);
+});