[google-auth-library-java] Flaky Test: ClientSideCredentialAccessBoundaryFactoryTest.refreshCredentialsIfRequired_asyncSingleThread

From: https://github.com/googleapis/google-auth-library-java/actions/runs/18476262098/job/52642249916?pr=1821

```
Error: Tests run: 29, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 2.029 s <<< FAILURE! -- in com.google.auth.credentialaccessboundary.ClientSideCredentialAccessBoundaryFactoryTest
Error: com.google.auth.credentialaccessboundary.ClientSideCredentialAccessBoundaryFactoryTest.refreshCredentialsIfRequired_asyncSingleThread -- Time elapsed: 0.019 s <<< FAILURE!
java.lang.AssertionError: expected:<1> but was:<2>
at org.junit.Assert.fail(Assert.java:89)
at org.junit.Assert.failNotEquals(Assert.java:835)
at org.junit.Assert.assertEquals(Assert.java:647)
at org.junit.Assert.assertEquals(Assert.java:633)
at com.google.auth.credentialaccessboundary.ClientSideCredentialAccessBoundaryFactoryTest.refreshCredentialsIfRequired_asyncSingleThread(ClientSideCredentialAccessBoundaryFactoryTest.java:295)
```

从第 295 行的 ClientSideCredentialAccessBoundaryFactoryTest.refreshCredentialsIfRequired_asyncSingleThread 开始，检查周围的异步凭据刷新设置。反复运行测试，以观察预期结果 1 与实际结果 2；当测试可靠且通过时不会出现间歇性失败，即表示完成。

由索引模型根据 Issue 内容生成。