Fix flaky int-test failure by making NettyClient response queue thread-safe

[Copilot]

Summary

This addresses the intermittent failure in Github Actions CI / int-test (pull_request) where FrontendIntegrationTest could throw IllegalStateException (Should have received response...) after request completion.
Root cause was cross-thread access to NettyClient.responseParts using a non-thread-safe LinkedList.

• Root cause

  • responseParts is produced on Netty I/O threads and consumed on test threads.
  • Using LinkedList could expose stale/empty reads under race, especially around channel close completion.

• Change

  • Replaced LinkedList<HttpObject> with ConcurrentLinkedQueue<HttpObject> in NettyClient.

• Effect

  • Preserves existing behavior while providing safe concurrent visibility for response parts, eliminating the observed flake path.

// NettyClient.java
private final Queue<HttpObject> responseParts = new ConcurrentLinkedQueue<HttpObject>();

Testing Done

Validated by re-running the previously failing frontend integration test path (multipartPostGetHeadUpdateDeleteUndeleteTest) and the containing FrontendIntegrationTest suite.