Polish async test by removing sleep in DefaultAsyncServerResponseTests

[hydrationn]

This PR removes Thread.sleep from async tests in DefaultAsyncServerResponseTests
and replaces it with a non-blocking CompletableFuture pattern. This makes the
test more deterministic and avoids timing-based flakiness.

🔍 What was changed

• Removed Thread.sleep(500) inside CompletableFuture.supplyAsync
• Replaced it with a non-blocking test pattern:
  • Created an incomplete CompletableFuture
  • Completed it asynchronously using new Thread(() -> future.complete(...))
• This makes the test more deterministic and avoids relying on timing-based behavior

💡 Why this change

Using Thread.sleep in tests is generally discouraged because:

• It increases test execution time
• It may lead to unstable test results depending on system timing
• It does not reflect real asynchronous behavior

The updated approach ensures:

• Faster test execution
• More reliable async behavior
• Clearer test intent

📌 Notes

No functional changes were made to production code.
This update improves the internal test quality and stability.