Fix retry-marker tooling breakages surfaced by CI

Three independent defects from the retry-marker work were failing pipeline
115477619 (98/377 tasks):

- RetryMarkerListener used System.err, violating forbiddenApis and failing
  :utils:junit-utils:forbiddenApisMain (check_base). Drop the log; the marker
  write is best-effort and must not fail the test run.
- The `dd.test.results.dir` system property set on every Test task tripped
  DDSpecification's strict system-property hygiene check (no non-allow-listed
  `dd.*`), turning every Spock spec into an initializationError. Move it out of
  the agent's product-config namespace to `datadog.test.results.dir`.
- JUnitReport.java used LinkedHashSet without importing it, breaking
  collect_results.sh in after_script (non-fatal, but result/marker collection
  silently produced nothing). Add the missing import.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mhdatie

.gitlab/collect-result/JUnitReport.java

@@ -4,6 +4,7 @@
 import java.nio.file.StandardCopyOption;
 import java.util.ArrayList;
 import java.util.LinkedHashMap;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;

buildSrc/src/main/kotlin/dd-trace-java.configure-tests.gradle.kts

@@ -49,7 +49,10 @@ tasks.withType<Test>().configureEach {
   // Trick to avoid on CI: "Couldn't flush user prefs: java.util.prefs.BackingStoreException: Couldn't get file lock."
   // Use a task-specific user prefs directory
   systemProperty("java.util.prefs.userRoot", layout.buildDirectory.dir("tmp/userPrefs/$name").get().asFile.absolutePath)
-  systemProperty("dd.test.results.dir", reports.junitXml.outputLocation.get().asFile.absolutePath)
+  // Test-infrastructure metadata for RetryMarkerListener. Deliberately NOT in the `dd.` namespace:
+  // that prefix is the agent's product-config namespace and is policed by DDSpecification's strict
+  // system-property hygiene check (setupSpec asserts no non-allow-listed `dd.*` property).
+  systemProperty("datadog.test.results.dir", reports.junitXml.outputLocation.get().asFile.absolutePath)
 
   // Enable JUnit 5 auto-detection so ConfigInversionExtension (STRICT mode) is loaded automatically
   systemProperty("junit.jupiter.extensions.autodetection.enabled", "true")

utils/junit-utils/src/main/java/datadog/trace/junit/utils/retry/RetryMarkerListener.java

@@ -22,7 +22,10 @@
 
 public class RetryMarkerListener implements TestExecutionListener {
 
-  static final String OUTPUT_DIR_PROP = "dd.test.results.dir";
+  // Not in the `dd.` namespace on purpose: that prefix is the agent's product-config namespace and
+  // is policed by DDSpecification's strict system-property hygiene check. Keep in sync with the
+  // `systemProperty(...)` set in dd-trace-java.configure-tests.gradle.kts.
+  static final String OUTPUT_DIR_PROP = "datadog.test.results.dir";
 
   private final Map<String, Integer> executionCounts = new ConcurrentHashMap<>();
   private final Map<String, TestIdentifier> identifiers = new ConcurrentHashMap<>();
@@ -48,7 +51,10 @@ public void testPlanExecutionFinished(TestPlan plan) {
         writeMarkerFile(outputDir, entry.getKey(), entry.getValue());
       }
     } catch (Exception ex) {
-      System.err.println("[RetryMarkerListener] Failed to write retry markers: " + ex.getMessage());
+      // Best-effort: a failed marker write only means a retried attempt may not be tagged as
+      // `skip`;
+      // never fail the test run over it. System.out/err is banned here by forbiddenApis, and the
+      // JUnit Platform launcher has no logger on this path, so swallow rather than log.
     }
   }