flaky test (datastore): DatastoreSampleApplicationIntegrationTests

Full [log](https://github.com/GoogleCloudPlatform/spring-cloud-gcp/actions/runs/15641608829/job/44070015864?pr=3848), error message:
```
Error: 4:858 [ERROR] Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 12.98 s <<< FAILURE! -- in com.example.DatastoreSampleApplicationIntegrationTests
Error: 4:859 [ERROR] com.example.DatastoreSampleApplicationIntegrationTests.runTests -- Time elapsed: 3.666 s <<< FAILURE!
org.opentest4j.AssertionFailedError:
[Verify Delete]
Expecting actual:
[Singer***singerId='null', firstName='John', lastName='Doe', albums=null, firstBand=null, bands=, personalInstruments=***,
Singer***singerId='null', firstName='Jane', lastName='Doe', albums=null, firstBand=null, bands=, personalInstruments=***,
Singer***singerId='null', firstName='Richard', lastName='Roe', albums=null, firstBand=null, bands=, personalInstruments=***]
to contain exactly (and in same order):
[Singer***singerId='null', firstName='Frodo', lastName='Baggins', albums=null, firstBand=null, bands=, personalInstruments=***,
Singer***singerId='null', firstName='Jane', lastName='Doe', albums=null, firstBand=null, bands=, personalInstruments=***,
Singer***singerId='null', firstName='Richard', lastName='Roe', albums=null, firstBand=null, bands=, personalInstruments=***]
but some elements were not found:
[Singer***singerId='null', firstName='Frodo', lastName='Baggins', albums=null, firstBand=null, bands=, personalInstruments=***]
and others were not expected:
[Singer***singerId='null', firstName='John', lastName='Doe', albums=null, firstBand=null, bands=, personalInstruments=***]

at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)
at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500)
at com.example.DatastoreSampleApplicationIntegrationTests.basicTest(DatastoreSampleApplicationIntegrationTests.java:138)
at com.example.DatastoreSampleApplicationIntegrationTests.runTests(DatastoreSampleApplicationIntegrationTests.java:94)
at java.base/java.lang.reflect.Method.invoke(Method.java:569)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
```

at glance, this seems a racing condition in test setup?

Start with DatastoreSampleApplicationIntegrationTests, especially basicTest at line 138 and runTests at line 94, then review the linked CI log. Reproduce the failure and determine whether test setup or datastore state ordering causes the mismatch; done means the integration test is deterministic and passes reliably.

Written by the indexing model from the issue text.