test: add tests for upgrade-sensitive areas

- Token model structure: verify $casts has datetime fields, $fillable
  is complete, and $table is set correctly (catches $dates→$casts regression)
- ClientServiceProvider: verify DeferrableProvider interface is
  implemented and provides() returns Client::class (catches $defer regression)
- Token parseOauthToken: verify stored values and Carbon types, not
  just that mock methods were called
- Client setToken: verify DataService cache is cleared on token change
- Client delegation: verify hasValidAccessToken/hasValidRefreshToken
  delegate to Token correctly

tests/TokenModelTest.php

@@ -0,0 +1,49 @@
+<?php
+
+use Illuminate\Contracts\Support\DeferrableProvider;
+use Wearepixel\QuickBooks\Client;
+use Wearepixel\QuickBooks\Providers\ClientServiceProvider;
+use Wearepixel\QuickBooks\Token;
+
+describe('Token model structure', function () {
+    it('uses the quickbooks_tokens table', function () {
+        expect((new Token)->getTable())->toBe('quickbooks_tokens');
+    });
+
+    it('casts expiration fields to datetime', function () {
+        $casts = (new Token)->getCasts();
+
+        expect($casts)
+            ->toHaveKey('access_token_expires_at', 'datetime')
+            ->toHaveKey('refresh_token_expires_at', 'datetime');
+    });
+
+    it('has all expected fillable attributes', function () {
+        $fillable = (new Token)->getFillable();
+
+        expect($fillable)->toEqualCanonicalizing([
+            'access_token',
+            'access_token_expires_at',
+            'realm_id',
+            'refresh_token',
+            'refresh_token_expires_at',
+            'user_id',
+        ]);
+    });
+});
+
+describe('ClientServiceProvider structure', function () {
+    it('implements DeferrableProvider', function () {
+        expect(ClientServiceProvider::class)
+            ->toImplement(DeferrableProvider::class);
+    });
+
+    it('declares Client in provides array', function () {
+        $app = Mockery::mock(\Illuminate\Contracts\Foundation\Application::class);
+        $app->shouldIgnoreMissing();
+
+        $provider = new ClientServiceProvider($app);
+
+        expect($provider->provides())->toBe([Client::class]);
+    });
+});