fix(appsec/jetty8): correct partPeek condition — use == 0 instead of == 1

CallDepthThreadLocalMap.incrementCallDepth() uses post-increment (depth++) so it returns
the depth BEFORE the increment. In the normal case (no GetPartAdvice active), Part.class
depth is 0 before the peek, making partPeek == 0. The previous guard checked partPeek == 1
which is true only when GetPartAdvice IS already on the stack — exactly backwards.
Result: GetFilenamesAdvice never fired for direct getParts() calls, breaking the filenames
event on Jetty 8.x.

Fix: use partPeek == 0 (proceed when GetPartAdvice is not active) so the condition is:
- partPeek == 0: normal direct getParts() → proceed (fire filenames event)
- partPeek == 1: getParts() called by getPart() in Jetty 9.0/9.1 → skip (GetPartAdvice handles it)

Author: jandro996

dd-java-agent/instrumentation/jetty/jetty-appsec/jetty-appsec-8.1.3/src/main/java/datadog/trace/instrumentation/jetty8/RequestGetPartsInstrumentation.java

@@ -127,12 +127,13 @@ static boolean before(
             final Object multiPartInputStream) {
       final int callDepth = CallDepthThreadLocalMap.incrementCallDepth(Collection.class);
       // _multiPartInputStream is null before the first parse; non-null on cached repeat calls.
-      // In Jetty 9.0/9.1, getPart(String) delegates to getParts() internally, which would
-      // trigger this advice and then GetPartAdvice — double-firing the filename event.
-      // Peek at Part.class depth: if > 1, GetPartAdvice is active and will handle the event.
+      // In Jetty 9.0/9.1, getPart(String) delegates to getParts() internally, triggering both
+      // GetPartAdvice and GetFilenamesAdvice — double-firing the filename event.
+      // incrementCallDepth returns the depth BEFORE incrementing (post-increment semantics).
+      // If Part.class depth is already 1, GetPartAdvice is active and will handle the event; skip.
       int partPeek = CallDepthThreadLocalMap.incrementCallDepth(Part.class);
       CallDepthThreadLocalMap.decrementCallDepth(Part.class);
-      return callDepth == 0 && multiPartInputStream == null && partPeek == 1;
+      return callDepth == 0 && multiPartInputStream == null && partPeek == 0;
     }
 
     @Advice.OnMethodExit(suppress = Throwable.class, onThrowable = Throwable.class)