Fix guarded CONTEXT_ATTRIBUTE_KEY removal in error and streaming paths

- handleLastHttpContent error path: use guarded removal (check identity
  before removing) to avoid orphaning the next keep-alive request's span
- handleHttpResponse: clear CONTEXT_ATTRIBUTE_KEY after moving context to
  STREAMING_CONTEXT_KEY to avoid double-processing in channelInactive

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

@@ -149,6 +149,7 @@ private void handleHttpResponse(
           && (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.
       }
     }
@@ -174,7 +175,9 @@ private void handleLastHttpContent(
         DECORATE.onError(span, throwable);
         span.setHttpStatusCode(500);
         span.finish();
-        ctx.channel().attr(CONTEXT_ATTRIBUTE_KEY).remove();
+        if (ctx.channel().attr(CONTEXT_ATTRIBUTE_KEY).get() == storedContext) {
+          ctx.channel().attr(CONTEXT_ATTRIBUTE_KEY).remove();
+        }
         throw throwable;
       }