[test] Unstable test FlinkUnionReadPrimaryKeyTableITCase.testUnionReadInStreamMode

https://github.com/apache/fluss/actions/runs/23418545308/job/68118735366

```
[ERROR] org.apache.fluss.lake.iceberg.flink.FlinkUnionReadPrimaryKeyTableITCase.testUnionReadInStreamMode(Boolean)[1] Time elapsed: 121.495 s <<< FAILURE!
java.lang.AssertionError: bucket TableBucket{tableId=2, 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.iceberg.testutils.FlinkIcebergTieringTestBase.waitUntilBucketSynced(FlinkIcebergTieringTestBase.java:525)
at org.apache.fluss.lake.iceberg.testutils.FlinkIcebergTieringTestBase.waitUntilBucketSynced(FlinkIcebergTieringTestBase.java:519)
at org.apache.fluss.lake.iceberg.flink.FlinkUnionReadPrimaryKeyTableITCase.testUnionReadInStreamMode(FlinkUnionReadPrimaryKeyTableITCase.java:78)
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)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:68)
... (truncated)
```

Start by reproducing FlinkUnionReadPrimaryKeyTableITCase.testUnionReadInStreamMode and inspect its failure at FlinkUnionReadPrimaryKeyTableITCase.java:78. Then read FlinkIcebergTieringTestBase.waitUntilBucketSynced and CommonTestUtils.waitUntil to trace why bucket TableBucket{tableId=2, bucket=0} is not synced. Done means the test completes reliably without the timeout assertion.

Written by the indexing model from the issue text.