fix(appsec): include filenames callback in Netty multipart early-return gate

requestFilesFilenames() was consulted after data collection, so when it was
the only registered AppSec callback (no requestBodyProcessed, no
requestFilesContent) the early-return gate fired and filenames were never
published to the WAF. Fetch all three callbacks upfront and gate on all three.

Author: jandro996

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

@@ -108,10 +108,13 @@ static void after(
       BiFunction<RequestContext, Object, Flow<Void>> callback =
           cbp.getCallback(EVENTS.requestBodyProcessed());
 
+      BiFunction<RequestContext, List<String>, Flow<Void>> filenamesCb =
+          cbp.getCallback(EVENTS.requestFilesFilenames());
+
       BiFunction<RequestContext, List<String>, Flow<Void>> contentCb =
           cbp.getCallback(EVENTS.requestFilesContent());
 
-      if (callback == null && contentCb == null) {
+      if (callback == null && filenamesCb == null && contentCb == null) {
         return;
       }
 
@@ -163,8 +166,6 @@ static void after(
       }
 
       if (!filenames.isEmpty()) {
-        BiFunction<RequestContext, List<String>, Flow<Void>> filenamesCb =
-            cbp.getCallback(EVENTS.requestFilesFilenames());
         if (filenamesCb != null) {
           Flow<Void> filenamesFlow = filenamesCb.apply(requestContext, filenames);
           Flow.Action filenamesAction = filenamesFlow.getAction();