[java-firestore] firestore.it.ITSystemTest.testRecursiveDeleteDoesNotAffectOtherCollections failed

observed in [pr](https://github.com/googleapis/java-firestore/pull/2130#issuecomment-2936869890), see failed [log](https://fusion2.corp.google.com/invocations/31f77978-9118-4e9e-8434-98c7036ce7c4/targets/github%2Fjava-firestore%2Fgoogle-cloud-firestore%2Ftarget%2Ffailsafe-reports%2Fintegration%2Fcom.google.cloud.firestore.it.ITSystemTest;config=default/tests) for details.

```
ITSystemTest(sponge_log).testRecursiveDeleteDoesNotAffectOtherCollectionscontent_copy | - | 2.286s
-- | -- | --
java.lang.AssertionError: expected:<0> but was:<1> at org.junit.Assert.fail(Assert.java:89) at org.junit.Assert.failNotEquals(Assert.java:835) at org.junit.Assert.assertEquals(Assert.java:647)

ITSystemTest(sponge_log).testRecursiveDeleteDoesNotAffectOtherCollections
- 2.286s
java.lang.AssertionError: expected:<0> but was:<1>
at org.junit.Assert.fail(Assert.java:89)
at org.junit.Assert.failNotEquals([Assert.java:835](https://cs.corp.google.com/piper///depot/google3/third_party/java_src/junit/main/java/org/junit/Assert.java?q=fail&l=835&ws&snapshot=0))
at org.junit.Assert.assertEquals([Assert.java:647](https://cs.corp.google.com/piper///depot/google3/third_party/java_src/junit/main/java/org/junit/Assert.java?q=failNotEquals&l=647&ws&snapshot=0))
```

Start with google-cloud-firestore's ITSystemTest.testRecursiveDeleteDoesNotAffectOtherCollections and the linked integration failure log. Trace the recursive-delete behavior around the reported assertion, then verify that the test passes while documents in other collections remain unaffected.

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