Fix GetFilenamesAdvice double-fire for Jetty 9.4+ where _multiParts replaces _contentParameters as the getParts() cache

In Jetty 9.3, getParts(MultiMap) sets _contentParameters, so the map==null guard prevents
re-firing on repeated getParts() calls. In Jetty 9.4+, getParts() delegates to getParts(null)
and caches the result in _multiParts instead, leaving _contentParameters null on every call.
Add _multiParts==null as an additional guard (optional=true handles Jetty 9.3 where the
field does not exist).

Author: jandro996

dd-java-agent/instrumentation/jetty/jetty-appsec/jetty-appsec-9.3/src/main/java/datadog/trace/instrumentation/jetty93/RequestExtractContentParametersInstrumentation.java

@@ -24,6 +24,7 @@
 import java.util.List;
 import java.util.function.BiFunction;
 import net.bytebuddy.asm.Advice;
+import net.bytebuddy.implementation.bytecode.assign.Assigner;
 import org.eclipse.jetty.server.Request;
 import org.eclipse.jetty.util.MultiMap;
 
@@ -117,9 +118,17 @@ static void after(
   @RequiresRequestContext(RequestContextSlot.APPSEC)
   public static class GetFilenamesAdvice {
     @Advice.OnMethodEnter(suppress = Throwable.class)
-    static boolean before(@Advice.FieldValue("_contentParameters") final MultiMap<String> map) {
+    static boolean before(
+        @Advice.FieldValue("_contentParameters") final MultiMap<String> contentParameters,
+        @Advice.FieldValue(value = "_multiParts", optional = true, typing = Assigner.Typing.DYNAMIC)
+            final Object multiParts) {
       final int callDepth = CallDepthThreadLocalMap.incrementCallDepth(Collection.class);
-      return callDepth == 0 && map == null;
+      // contentParameters is set by extractContentParameters() (called from getParameterMap()),
+      // so it being non-null means the request was already processed via that path.
+      // multiParts is set by getParts(MultiMap) (Jetty 9.4+) after the first getParts() call,
+      // so it being non-null means getParts() was already invoked and filenames were reported.
+      // In Jetty 9.3, _multiParts does not exist (optional=true → null).
+      return callDepth == 0 && contentParameters == null && multiParts == null;
     }
 
     @Advice.OnMethodExit(suppress = Throwable.class, onThrowable = Throwable.class)