[test] Unstable test CoordinatorEventProcessorTest.testDoBucketReassignment

https://github.com/apache/fluss/actions/runs/23795675993/job/69342199651

```
[ERROR] org.apache.fluss.server.coordinator.CoordinatorEventProcessorTest.testDoBucketReassignment Time elapsed: 60.027 s <<< FAILURE!
java.lang.AssertionError: leader not elected
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.testutils.common.CommonTestUtils.waitValue(CommonTestUtils.java:113)
at org.apache.fluss.server.coordinator.CoordinatorEventProcessorTest.verifyIsr(CoordinatorEventProcessorTest.java:1045)
at org.apache.fluss.server.coordinator.CoordinatorEventProcessorTest.testDoBucketReassignment(CoordinatorEventProcessorTest.java:1009)
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$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:156)
at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:147)
at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:86)
at org.junit.jupiter.engine.execution.InterceptingExecutableInvoker$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 with CoordinatorEventProcessorTest.testDoBucketReassignment, especially verifyIsr at line 1045 and the waitValue/waitUntil calls in CommonTestUtils. Reproduce the failure from the linked GitHub Actions run and trace why leader election does not complete within the timeout. Done means the test reliably passes without masking the underlying coordination failure.

Written by the indexing model from the issue text.