Make PushError implement JsonSerializable (#85)

`PushError` now implements `JsonSerializable` via the existing
`ConvertsToJson` trait, so instances can be passed directly to
`json_encode()` or converted with `toJson()`.

Closes #74

Author: dru1x

src/PushError/PushError.php

@@ -2,8 +2,13 @@
 
 namespace Dru1x\ExpoPush\PushError;
 
-final readonly class PushError
+use Dru1x\ExpoPush\Support\ConvertsToJson;
+use JsonSerializable;
+
+final readonly class PushError implements JsonSerializable
 {
+    use ConvertsToJson;
+
     /**
      * @param ?array<string, mixed> $details
      */

tests/Unit/PushError/PushErrorTest.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace Dru1x\ExpoPush\Tests\Unit\PushError;
+
+use Dru1x\ExpoPush\PushError\PushError;
+use Dru1x\ExpoPush\PushError\PushErrorCode;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+class PushErrorTest extends TestCase
+{
+    #[Test]
+    public function json_encode_returns_value(): void
+    {
+        $error = new PushError(
+            code: PushErrorCode::PushTooManyNotifications,
+            message: 'Too many notifications',
+            details: ['field' => 'to'],
+            startIndex: 0,
+            endIndex: 5,
+        );
+
+        $expectedJson = <<<JSON
+            {
+                "code": "PUSH_TOO_MANY_NOTIFICATIONS",
+                "message": "Too many notifications",
+                "details": {"field": "to"},
+                "startIndex": 0,
+                "endIndex": 5
+            }
+            JSON;
+
+        $this->assertJsonStringEqualsJsonString($expectedJson, json_encode($error));
+    }
+
+    #[Test]
+    public function to_json_returns_value(): void
+    {
+        $error = new PushError(
+            code: PushErrorCode::PushTooManyNotifications,
+            message: 'Too many notifications',
+            details: ['field' => 'to'],
+            startIndex: 0,
+            endIndex: 5,
+        );
+
+        $expectedJson = <<<JSON
+            {
+                "code": "PUSH_TOO_MANY_NOTIFICATIONS",
+                "message": "Too many notifications",
+                "details": {"field": "to"},
+                "startIndex": 0,
+                "endIndex": 5
+            }
+            JSON;
+
+        $this->assertJsonStringEqualsJsonString($expectedJson, $error->toJson());
+    }
+}