test: strengthen assertions and add edge-case content negotiation tests

Add tests for JSON Content-Type with charset parameter, vendor +json
suffix, and case-insensitive Content-Type matching through the
responseContentType parameter.

Add trait-level test for JSON response on mixed-content-type endpoint.
Strengthen assertion specificity in content type mismatch tests.

Author: wadakatu

tests/Unit/OpenApiResponseValidatorTest.php

@@ -369,8 +369,10 @@ public function v30_response_content_type_not_in_spec_fails(): void
         );
 
         $this->assertFalse($result->isValid());
-        $this->assertStringContainsString('not defined', $result->errors()[0]);
-        $this->assertStringContainsString('text/plain', $result->errors()[0]);
+        $this->assertStringContainsString("Response Content-Type 'text/plain' is not defined for", $result->errors()[0]);
+        $this->assertStringContainsString('text/html', $result->errors()[0]);
+        $this->assertStringContainsString('application/json', $result->errors()[0]);
+        $this->assertSame('/v1/pets', $result->matchedPath());
     }
 
     #[Test]
@@ -389,6 +391,75 @@ public function v30_response_content_type_with_charset_matches_spec(): void
         $this->assertSame('/v1/pets', $result->matchedPath());
     }
 
+    #[Test]
+    public function v30_json_response_content_type_with_charset_validates_schema(): void
+    {
+        $result = $this->validator->validate(
+            'petstore-3.0',
+            'POST',
+            '/v1/pets',
+            409,
+            ['error' => 'Pet already exists'],
+            'application/json; charset=utf-8',
+        );
+
+        $this->assertTrue($result->isValid());
+        $this->assertSame('/v1/pets', $result->matchedPath());
+    }
+
+    #[Test]
+    public function v30_json_response_content_type_with_charset_and_invalid_body_fails(): void
+    {
+        $result = $this->validator->validate(
+            'petstore-3.0',
+            'POST',
+            '/v1/pets',
+            409,
+            ['wrong_key' => 'value'],
+            'application/json; charset=utf-8',
+        );
+
+        $this->assertFalse($result->isValid());
+        $this->assertNotEmpty($result->errors());
+    }
+
+    #[Test]
+    public function v30_vendor_json_response_content_type_validates_schema(): void
+    {
+        $result = $this->validator->validate(
+            'petstore-3.0',
+            'GET',
+            '/v1/pets',
+            400,
+            [
+                'type' => 'https://example.com/bad-request',
+                'title' => 'Bad Request',
+                'status' => 400,
+                'detail' => 'Invalid query parameter',
+            ],
+            'application/problem+json',
+        );
+
+        $this->assertTrue($result->isValid());
+        $this->assertSame('/v1/pets', $result->matchedPath());
+    }
+
+    #[Test]
+    public function v30_case_insensitive_response_content_type_matches_spec(): void
+    {
+        $result = $this->validator->validate(
+            'petstore-3.0',
+            'POST',
+            '/v1/pets',
+            409,
+            null,
+            'Text/HTML',
+        );
+
+        $this->assertTrue($result->isValid());
+        $this->assertSame('/v1/pets', $result->matchedPath());
+    }
+
     #[Test]
     public function v30_null_response_content_type_preserves_existing_behavior(): void
     {

tests/Unit/ValidatesOpenApiSchemaTest.php

@@ -169,7 +169,7 @@ public function non_json_body_fails_with_content_type_mismatch(): void
         );
 
         $this->expectException(AssertionFailedError::class);
-        $this->expectExceptionMessage('not defined');
+        $this->expectExceptionMessage("Response Content-Type 'text/html' is not defined for");
 
         $this->assertResponseMatchesOpenApiSchema(
             $response,
@@ -242,6 +242,19 @@ public function missing_content_type_header_still_parses_json(): void
         );
     }
 
+    #[Test]
+    public function json_content_type_in_spec_with_mixed_content_types_validates_schema(): void
+    {
+        $body = (string) json_encode(['error' => 'Pet already exists'], JSON_THROW_ON_ERROR);
+        $response = $this->makeTestResponse($body, 409, ['Content-Type' => 'application/json']);
+
+        $this->assertResponseMatchesOpenApiSchema(
+            $response,
+            HttpMethod::POST,
+            '/v1/pets',
+        );
+    }
+
     #[Test]
     public function non_json_content_type_in_spec_with_mixed_content_types_passes(): void
     {