[test] Unstable test FlussAuthorizationITCase. testInitWriter

### 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/21698664388/job/62574270578

```
[INFO] Running org.apache.fluss.client.table.FlussTableITCase
Error: Tests run: 22, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 181.549 s <<< FAILURE! - in org.apache.fluss.client.security.acl.FlussAuthorizationITCase
Error: org.apache.fluss.client.security.acl.FlussAuthorizationITCase.testInitWriter Time elapsed: 71.391 s <<< FAILURE!
java.lang.AssertionError:

Expecting Optional to contain a value but it was empty.
at org.apache.fluss.server.testutils.FlussClusterExtension.waitReplicaInAssignmentReady(FlussClusterExtension.java:619)
at org.apache.fluss.server.testutils.FlussClusterExtension.lambda$waitUntilTableReady$8(FlussClusterExtension.java:558)
at org.apache.fluss.testutils.common.CommonTestUtils.retry(CommonTestUtils.java:142)
at org.apache.fluss.server.testutils.FlussClusterExtension.waitUntilTableReady(FlussClusterExtension.java:552)
at org.apache.fluss.client.security.acl.FlussAuthorizationITCase.testInitWriter(FlussAuthorizationITCase.java:549)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:727)
at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain$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)
```

### Solution

_No response_

### Are you willing to submit a PR?

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

Start with FlussAuthorizationITCase.testInitWriter at line 549 and the failure path in FlussClusterExtension.waitUntilTableReady and waitReplicaInAssignmentReady at lines 552-619. Run the failing integration test and investigate why the replica assignment remains empty during initialization. Done means testInitWriter passes reliably without the Optional assertion failure.

Written by the indexing model from the issue text.