Fix websocket upgrade span leak in handleHttpResponse, improve test docs

- WebSocket upgrades in handleHttpResponse now finish the span immediately
  instead of deferring to LastHttpContent (which never arrives for websockets)
- Updated test Javadocs to accurately describe what each test validates
- Improved connection-drop test synchronization (read first byte instead of sleep)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gtukmachev

dd-java-agent/instrumentation/netty/netty-4.1/src/main/java/datadog/trace/instrumentation/netty41/server/HttpServerResponseTracingHandler.java

@@ -148,9 +148,17 @@ private void handleHttpResponse(
       if (response.status() != HttpResponseStatus.CONTINUE
           && (response.status() != HttpResponseStatus.SWITCHING_PROTOCOLS || isWebsocketUpgrade)) {
         DECORATE.onResponse(span, response);
-        ctx.channel().attr(STREAMING_CONTEXT_KEY).set(storedContext);
-        ctx.channel().attr(CONTEXT_ATTRIBUTE_KEY).remove();
-        // Span finish is deferred to handleLastHttpContent.
+
+        if (isWebsocketUpgrade) {
+          // WebSocket upgrades don't produce LastHttpContent — finish span immediately.
+          DECORATE.beforeFinish(scope.context());
+          span.finish();
+          ctx.channel().attr(CONTEXT_ATTRIBUTE_KEY).remove();
+        } else {
+          ctx.channel().attr(STREAMING_CONTEXT_KEY).set(storedContext);
+          ctx.channel().attr(CONTEXT_ATTRIBUTE_KEY).remove();
+          // Span finish is deferred to handleLastHttpContent.
+        }
       }
     }
   }