[CI] Stabilize Netty client disconnection test

[litiliu]

• No generative AI tools used
• Yes (Codex)

Generated-by: Codex following the Apache Fluss guidelines

Purpose

Linked issue: close #4346

NettyClientTest.testServerDisconnection immediately checks the connection map after a failed request completes. The request future can complete before the asynchronous close callback removes the disconnected connection, making the test flaky with expected: 0 but was: 1.

Brief change log

• Retry the existing AssertJ assertion until the asynchronous connection cleanup completes.
• Keep production connection behavior unchanged.

Tests

• ./mvnw -pl fluss-rpc -am -DskipITs test
• 10 consecutive runs of NettyClientTest#testServerDisconnection
• ./mvnw -pl fluss-rpc spotless:check checkstyle:check

API and Format

No API or storage format changes.

Documentation

No documentation changes.