Extract filename extraction to MultipartHelper in jetty-appsec-9.2 + unit tests

Move the getSubmittedFileName() loop from GetFilenamesAdvice into a new
MultipartHelper helper class (injected via helperClassNames) so it can be
unit tested in isolation. Add 8 Spock test cases covering null/empty
collections, null/empty filenames, multiple parts, and special characters.

Author: jandro996

dd-java-agent/instrumentation/jetty/jetty-appsec/jetty-appsec-9.2/src/main/java/datadog/trace/instrumentation/jetty92/MultipartHelper.java

@@ -0,0 +1,32 @@
+package datadog.trace.instrumentation.jetty92;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+import javax.servlet.http.Part;
+
+public class MultipartHelper {
+
+  private MultipartHelper() {}
+
+  /**
+   * Extracts non-null, non-empty filenames from a collection of multipart {@link Part}s using
+   * {@link Part#getSubmittedFileName()} (Servlet 3.1+).
+   *
+   * @return list of filenames; never {@code null}, may be empty
+   */
+  public static List<String> extractFilenames(Collection<Part> parts) {
+    if (parts == null || parts.isEmpty()) {
+      return Collections.emptyList();
+    }
+    List<String> filenames = new ArrayList<>();
+    for (Part part : parts) {
+      String name = part.getSubmittedFileName();
+      if (name != null && !name.isEmpty()) {
+        filenames.add(name);
+      }
+    }
+    return filenames;
+  }
+}

dd-java-agent/instrumentation/jetty/jetty-appsec/jetty-appsec-9.2/src/main/java/datadog/trace/instrumentation/jetty92/RequestExtractContentParametersInstrumentation.java

@@ -19,7 +19,6 @@
 import datadog.trace.api.gateway.RequestContextSlot;
 import datadog.trace.bootstrap.CallDepthThreadLocalMap;
 import datadog.trace.bootstrap.instrumentation.api.AgentTracer;
-import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
 import java.util.function.BiFunction;
@@ -42,6 +41,11 @@ public String instrumentedType() {
     return "org.eclipse.jetty.server.Request";
   }
 
+  @Override
+  public String[] helperClassNames() {
+    return new String[] {packageName + ".MultipartHelper"};
+  }
+
   @Override
   public void methodAdvice(MethodTransformer transformer) {
     transformer.applyAdvice(
@@ -159,13 +163,7 @@ static void after(
       if (!proceed || t != null || parts == null || parts.isEmpty()) {
         return;
       }
-      List<String> filenames = new ArrayList<>();
-      for (Part part : parts) {
-        String name = part.getSubmittedFileName();
-        if (name != null && !name.isEmpty()) {
-          filenames.add(name);
-        }
-      }
+      List<String> filenames = MultipartHelper.extractFilenames(parts);
       if (filenames.isEmpty()) {
         return;
       }

dd-java-agent/instrumentation/jetty/jetty-appsec/jetty-appsec-9.2/src/test/groovy/datadog/trace/instrumentation/jetty92/MultipartHelperTest.groovy

@@ -0,0 +1,71 @@
+package datadog.trace.instrumentation.jetty92
+
+import javax.servlet.http.Part
+import spock.lang.Specification
+
+class MultipartHelperTest extends Specification {
+
+  def "returns empty list for null collection"() {
+    expect:
+    MultipartHelper.extractFilenames(null) == []
+  }
+
+  def "returns empty list for empty collection"() {
+    expect:
+    MultipartHelper.extractFilenames([]) == []
+  }
+
+  def "returns empty list when all parts have null filename"() {
+    given:
+    def parts = [part(null), part(null)]
+
+    expect:
+    MultipartHelper.extractFilenames(parts) == []
+  }
+
+  def "returns empty list when all parts have empty filename"() {
+    given:
+    def parts = [part(''), part('')]
+
+    expect:
+    MultipartHelper.extractFilenames(parts) == []
+  }
+
+  def "extracts filename from single part"() {
+    given:
+    def parts = [part('photo.jpg')]
+
+    expect:
+    MultipartHelper.extractFilenames(parts) == ['photo.jpg']
+  }
+
+  def "extracts filenames from multiple parts"() {
+    given:
+    def parts = [part('a.jpg'), part('b.png'), part('c.pdf')]
+
+    expect:
+    MultipartHelper.extractFilenames(parts) == ['a.jpg', 'b.png', 'c.pdf']
+  }
+
+  def "skips parts with null or empty filename and keeps valid ones"() {
+    given:
+    def parts = [part(null), part('valid.txt'), part(''), part('other.zip')]
+
+    expect:
+    MultipartHelper.extractFilenames(parts) == ['valid.txt', 'other.zip']
+  }
+
+  def "preserves filenames with spaces and special characters"() {
+    given:
+    def parts = [part('my file.tar.gz'), part('résumé.pdf')]
+
+    expect:
+    MultipartHelper.extractFilenames(parts) == ['my file.tar.gz', 'résumé.pdf']
+  }
+
+  private Part part(String submittedFileName) {
+    Part p = Stub(Part)
+    p.getSubmittedFileName() >> submittedFileName
+    return p
+  }
+}