Flaky Test: PartitionBalanceCoordinatorGrpcTest

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/incubator-uniffle/issues?q=is%3Aissue) and found no similar issues.

### Describe the flaky test

```
Error: Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 4.239 s <<< FAILURE! - in org.apache.uniffle.test.PartitionBalanceCoordinatorGrpcTest
Error: getShuffleAssignmentsTest Time elapsed: 3.24 s <<< FAILURE!
org.opentest4j.AssertionFailedError: expected: <20001> but was: <20002>
at org.junit.jupiter.api.AssertionUtils.fail(AssertionUtils.java:55)
at org.junit.jupiter.api.AssertionUtils.failNotEqual(AssertionUtils.java:62)
at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:150)
at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:145)
at org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:527)
at org.apache.uniffle.test.PartitionBalanceCoordinatorGrpcTest.getShuffleAssignmentsTest(PartitionBalanceCoordinatorGrpcTest.java:98)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:725)
at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:149)
at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:140)
at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:84)
at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(ExecutableInvoker.java:115)
at org.junit.jupiter.engine.execution.ExecutableInvoker.lambda$invoke$0(ExecutableInvoker.java:105)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain$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.ExecutableInvoker.invoke(ExecutableInvoker.java:104)
at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:98)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$7(TestMethodTestDescriptor.java:214)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:210)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:135)
```

### Actions URL

https://github.com/apache/incubator-uniffle/actions/runs/3570753180/jobs/6002010978
https://github.com/apache/incubator-uniffle/actions/runs/4516940829/jobs/7955749162?pr=759
https://github.com/apache/incubator-uniffle/actions/runs/5517393179/jobs/10059951405?pr=1002
### Parent issue

#1733

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

Start by rerunning PartitionBalanceCoordinatorGrpcTest and focus on getShuffleAssignmentsTest at line 98, where the expected 20001 value differs from the observed 20002. Trace the test setup and assignment result to identify the source of nondeterminism; done means the test passes consistently without intermittent assignment-count failures.

Written by the indexing model from the issue text.