fix(entity): nullable型の戻り値型エラーを修正

PHPUnitテストで発生していた型エラーを修正:

- PaymentResult::getResponse(): Response|null に変更し、nullチェックを追加
- Tag::getName(): string|null に変更し、__toString()でnull対応
- Order::getUsePoint(): string|null に変更（PointTrait/ItemHolderInterface）
- HelpControllerTest: デバッグ用のエラーメッセージを追加

これにより以下のテストエラーが解消:
- ShoppingControllerTest::testCompleteWithLogin 他7件（PaymentResult）
- TagControllerTest::testEditInvalid（Tag::getName）
- ShoppingControllerTest::testPaymentWithError（Order::getUsePoint）

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: nanasess

src/Eccube/Controller/ShoppingController.php

@@ -938,7 +938,7 @@ protected function executeCheckout(PaymentMethodInterface $paymentMethod): \Symf
         $PaymentResult = $paymentMethod->checkout();
         $response = $PaymentResult->getResponse();
         // PaymentResultがresponseを保持している場合はresponseを返す
-        if ($response->isRedirection() || $response->isSuccessful()) {
+        if ($response && ($response->isRedirection() || $response->isSuccessful())) {
             $this->entityManager->flush();
             log_info('[注文処理] PaymentMethod::checkoutが指定したレスポンスを表示します.');
 

src/Eccube/Entity/ItemHolderInterface.php

@@ -119,9 +119,9 @@ public function setUsePoint($usePoint): static;
     /**
      * 利用ポイントを返します.
      *
-     * @return string
+     * @return string|null
      */
-    public function getUsePoint(): string;
+    public function getUsePoint(): ?string;
 
     /**
      * @param ItemInterface $item

src/Eccube/Entity/PointTrait.php

@@ -72,9 +72,9 @@ public function setUsePoint($usePoint): static
     /**
      * Get usePoint
      *
-     * @return string
+     * @return string|null
      */
-    public function getUsePoint(): string
+    public function getUsePoint(): ?string
     {
         return $this->use_point;
     }

src/Eccube/Entity/Tag.php

@@ -39,7 +39,7 @@ class Tag extends AbstractEntity implements \Stringable
         #[\Override]
         public function __toString(): string
         {
-            return $this->getName();
+            return $this->getName() ?? '';
         }
 
         /**
@@ -123,9 +123,9 @@ public function setName($name): static
         /**
          * Get name.
          *
-         * @return string
+         * @return string|null
          */
-        public function getName(): string
+        public function getName(): ?string
         {
             return $this->name;
         }

src/Eccube/Service/Payment/PaymentResult.php

@@ -106,9 +106,9 @@ public function setResponse(Response $response): PaymentResult
     /**
      * Response を返します.
      *
-     * @return Response
+     * @return Response|null
      */
-    public function getResponse(): Response
+    public function getResponse(): ?Response
     {
         return $this->response;
     }

tests/Eccube/Tests/Web/HelpControllerTest.php

@@ -22,7 +22,8 @@ public function testRoutingHelpTradelaw()
     {
         $client = $this->client;
         $client->request('GET', $this->generateUrl('help_tradelaw'));
-        $this->assertTrue($client->getResponse()->isSuccessful());
+        $response = $client->getResponse();
+        $this->assertTrue($response->isSuccessful(), 'Response status: '.$response->getStatusCode().' - Content: '.$response->getContent());
     }
 
     /**