[test] Unstable test FlinkUnionReadPrimaryKeyTableITCase.testUnionReadInStreamMode

https://github.com/apache/fluss/actions/runs/27193285127/job/80278508642

```
[ERROR] org.apache.fluss.lake.paimon.flink.FlinkUnionReadPrimaryKeyTableITCase.testUnionReadInStreamMode(Boolean)[2] Time elapsed: 121.66 s <<< FAILURE!
java.lang.AssertionError: bucket TableBucket{tableId=12, partitionId=10, 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.FlinkUnionReadPrimaryKeyTableITCase.testUnionReadInStreamMode(FlinkUnionReadPrimaryKeyTableITCase.java:564)
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$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:156)
at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:147)
at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestTemplateMethod(TimeoutExtension.java:94)
at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(InterceptingExecutableInvoker.java:103)
at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.lambda$invoke$0(InterceptingExecutableInvoker.java:93)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:106)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:64)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:45)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:37)
at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.invoke(InterceptingExecutableInvoker.java:92)
at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker.invoke(InterceptingExecutableInvoker.java:86)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$7(TestMethodTestDescriptor.java:217)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:213)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:138)
... (truncated)
```

Start with FlinkUnionReadPrimaryKeyTableITCase.testUnionReadInStreamMode at the failing line 564 and review the linked CI failure. Trace the waitUntilBucketSynced calls through FlinkPaimonTieringTestBase and CommonTestUtils.waitUntil to determine why the bucket remains unsynced. Done means the integration test completes reliably in stream mode without the synchronization timeout.

Written by the indexing model from the issue text.