Declare the billable's API caches, so Eloquent models can still be saved

The caches were undeclared properties: on an Eloquent model they went through
Model::__set into the attributes, and save() failed with "no such column: ordersCache".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/Id/BillableModelTest.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace Anikeen\Id\Tests;
+
+use Anikeen\Id\Billable;
+use Illuminate\Database\Eloquent\Model;
+
+class BillableModelTest extends TestCase
+{
+    public function test_cached_api_results_are_not_model_attributes(): void
+    {
+        $user = new class extends Model {
+            use Billable;
+
+            protected $guarded = [];
+
+            public function cacheAll(): void
+            {
+                foreach (['addresses', 'countries', 'defaultPaymentMethod', 'invoices', 'orders', 'paymentMethods', 'subscriptions', 'transactions', 'userData'] as $name) {
+                    $this->{$name.'Cache'} = 'cached';
+                }
+            }
+        };
+        $user->email = 'ada@example.com';
+
+        $user->cacheAll();
+
+        // save() would otherwise write them as columns ("no such column: ordersCache")
+        $this->assertSame(['email' => 'ada@example.com'], $user->getAttributes());
+    }
+}