Should we use mock server in test?

I got these errors while running tests via PHPUnit locally.

```
2) ReferenceTest::testResolveFileHttp
cebe\openapi\exceptions\UnresolvableReferenceException: Failed to resolve Reference 'definitions.yaml#/Pet' to cebe\openapi\spec\Schema Object: file_get_contents(https://raw.githubusercontent.com/cebe/php-openapi/290389bbd337cf4d70ecedfd3a3d886715e19552/tests/spec/data/reference/definitions.yaml): failed to open stream: HTTP request failed! HTTP/1.1 429 Too Many Requests

/app/src/spec/Reference.php:226
/app/src/SpecBaseObject.php:416
/app/src/SpecBaseObject.php:412
/app/tests/spec/ReferenceTest.php:247
phpvfscomposer:///app/vendor/phpunit/phpunit/phpunit:106

Caused by
PHPUnit\Framework\Error\Warning: file_get_contents(https://raw.githubusercontent.com/cebe/php-openapi/290389bbd337cf4d70ecedfd3a3d886715e19552/tests/spec/data/reference/definitions.yaml): failed to open stream: HTTP request failed! HTTP/1.1 429 Too Many Requests

/app/src/ReferenceContext.php:217
/app/src/spec/Reference.php:224
/app/src/SpecBaseObject.php:416
/app/src/SpecBaseObject.php:412
/app/tests/spec/ReferenceTest.php:247
phpvfscomposer:///app/vendor/phpunit/phpunit/phpunit:106

```

Should we use mock server to fix this issue?

从 tests/spec/ReferenceTest.php 开始，尤其是 testResolveFileHttp，并通过 src/spec/Reference.php 和 src/ReferenceContext.php 追踪故障。确定 fixture 如何避免 raw.githubusercontent.com 的速率限制，然后运行 PHPUnit 测试，以验证它不再依赖该外部请求。

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