[test] Unstable test FlinkUnionReadLogTableITCase.testReadLogTableInStreamMode

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Fluss version

main (development)

### Please describe the bug 🐞

https://github.com/apache/fluss/actions/runs/21700629651/job/62580213598

```
Error: Tests run: 7, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 220.058 s <<< FAILURE! - in org.apache.fluss.lake.paimon.flink.FlinkUnionReadLogTableITCase
Error: org.apache.fluss.lake.paimon.flink.FlinkUnionReadLogTableITCase.testReadLogTableInStreamMode(boolean)[2] Time elapsed: 121.961 s <<< FAILURE!
java.lang.AssertionError: bucket TableBucket{tableId=8, partitionId=9, bucket=0}not synced
at org.apache.fluss.testutils.common.CommonTestUtils.waitUntil(CommonTestUtils.java:81)
at org.apache.fluss.testutils.common.CommonTestUtils.waitUntil(CommonTestUtils.java:99)
at org.apache.fluss.lake.paimon.testutils.FlinkPaimonTieringTestBase.waitUntilBucketSynced(FlinkPaimonTieringTestBase.java:442)
at org.apache.fluss.lake.paimon.testutils.FlinkPaimonTieringTestBase.waitUntilBucketsSynced(FlinkPaimonTieringTestBase.java:437)
at org.apache.fluss.lake.paimon.testutils.FlinkPaimonTieringTestBase.waitUntilBucketSynced(FlinkPaimonTieringTestBase.java:432)
at org.apache.fluss.lake.paimon.flink.FlinkUnionReadLogTableITCase.testReadLogTableInStreamMode(FlinkUnionReadLogTableITCase.java:156)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:727)
at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:156)
at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:147)
at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestTemplateMethod(TimeoutExtension.java:94)
at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker$ReflectiveI
```

### Solution

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start with FlinkUnionReadLogTableITCase.java at testReadLogTableInStreamMode, especially line 156, then trace waitUntilBucketSynced through FlinkPaimonTieringTestBase.java lines 432-442 and CommonTestUtils.java lines 81 and 99. Reproduce the test and investigate why bucket TableBucket{tableId=8, partitionId=9, bucket=0} is not synced; done means the test completes reliably without the timeout assertion.

Written by the indexing model from the issue text.