✨ Handle non-numeric values in selected filter for Country API and add corresponding tests

- 🔧 Refactor `CountryController` to safely process non-numeric values in `selected` query parameter using `array_filter`.
- 🧪 Add feature test to ensure API does not crash when `selected` includes non-numeric codes.

tests/Feature/Api/CountryControllerTest.php

@@ -0,0 +1,13 @@
+<?php
+
+use App\Models\Country;
+
+it('does not crash when selected contains non-numeric codes', function () {
+    Country::factory()->create(['code' => 'CH']);
+
+    $response = $this->getJson('/api/countries?'.http_build_query([
+        'selected' => ['CH', 'de', '1'],
+    ]));
+
+    $response->assertSuccessful();
+});