flaky test: FirestoreSampleAppIntegrationTests

test failed in native build due to Deadlocked threads

see full log [here](https://github.com/GoogleCloudPlatform/spring-cloud-gcp/actions/runs/15935307683/job/44953697284?pr=3902)

```
Failures (1):
JUnit Jupiter:FirestoreSampleAppIntegrationTests:testSample()
MethodSource [className = 'com.example.FirestoreSampleAppIntegrationTests', methodName = 'testSample', methodParameterTypes = '']
=> org.awaitility.core.ConditionTimeoutException: Condition with Lambda expression in com.example.FirestoreSampleAppIntegrationTests was not fulfilled within 10 seconds.
org.awaitility.core.ConditionAwaiter.await(ConditionAwaiter.java:167)
org.awaitility.core.CallableCondition.await(CallableCondition.java:78)
org.awaitility.core.CallableCondition.await(CallableCondition.java:26)
org.awaitility.core.ConditionFactory.until(ConditionFactory.java:1160)
org.awaitility.core.ConditionFactory.until(ConditionFactory.java:1129)
[...]
Caused by: org.awaitility.core.DeadlockException: Deadlocked threads detected:
```