fix: remove static ExecutorService in AsyncFeign to prevent ClassLoader leak

[anuragg-saxenaa]

Problem

AsyncFeign held a static singleton ExecutorService via LazyInitializedExecutorService:

private static class LazyInitializedExecutorService {
  private static final ExecutorService instance =
      Executors.newCachedThreadPool(r -> {
        final Thread result = new Thread(r);
        result.setDaemon(true);
        return result;
      });
}

In servlet containers (Tomcat, Jetty, etc.), redeploying an application does not restart the JVM. The static thread pool holds a strong reference to the ContextClassLoader of the first deployment. On each redeployment:

• The old ClassLoader is never GC'd (retained by the static thread pool's threads)
• Thread count grows linearly
• Metaspace eventually exhausts → OutOfMemoryError

Setting threads as daemon only helps at JVM exit — it does not prevent ClassLoader leaks during hot-redeploy.

Fix

Remove LazyInitializedExecutorService entirely. AsyncBuilder.client is now initialized with a fresh Executors.newCachedThreadPool(...) per builder instance. This means:

• Each AsyncFeign.builder() creates its own independent executor
• When the client is discarded, the executor becomes eligible for GC
• Users who need lifecycle control or sharing can supply their own via .client(AsyncClient)

Testing

Added asyncFeignHasNoStaticExecutorService() to AsyncFeignTest — uses reflection to assert that no static field of type ExecutorService exists in any inner class of AsyncFeign.

Fixes #3178

[yvasyliev]

isn't it already covered in #3277?

[anuragg-saxenaa]

Closing as duplicate of #3277 which addresses the same issue with a more complete fix (also covering CoroutineFeign). Thanks for the pointer @yvasyliev!