feat(virtual-thread): Use context swapping instead of scope activation

Replaces the `ConcurrentState` approach (which activated/closed individual scopes) with `VirtualThreadState` that swaps the entire scope stack via `Context#swap()`. This correctly handles child spans created during virtual thread execution and avoids out-of-order scope closing. The new approach mirrors the ZIO FiberContext and Kotlin coroutines instrumentation patterns.

Author: PerfectSlayer

dd-java-agent/instrumentation-testing/src/main/java/datadog/trace/agent/test/AbstractInstrumentationTest.java

@@ -30,6 +30,7 @@
 import java.util.function.Predicate;
 import net.bytebuddy.agent.ByteBuddyAgent;
 import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.opentest4j.AssertionFailedError;
@@ -42,13 +43,6 @@
  */
 @ExtendWith(TestClassShadowingExtension.class)
 public abstract class AbstractInstrumentationTest {
-  static {
-    // Allow re-registration of ContextManagers so each test can use a fresh tracer.
-    // This mirrors DDSpecification.allowContextTesting() for the JUnit 5 test framework.
-    datadog.context.ContextManager.allowTesting();
-    datadog.context.ContextBinder.allowTesting();
-  }
-
   static final Instrumentation INSTRUMENTATION = ByteBuddyAgent.getInstrumentation();
 
   static final long TIMEOUT_MILLIS = TimeUnit.SECONDS.toMillis(20);
@@ -60,6 +54,18 @@ public abstract class AbstractInstrumentationTest {
   protected ClassFileTransformer activeTransformer;
   protected ClassFileTransformerListener transformerLister;
 
+  @BeforeAll
+  static void allowContextTesting() {
+    // Allow re-registration of context managers so each test can use a fresh tracer.
+    // This mirrors DDSpecification.allowContextTesting() for the Spock test framework.
+    try {
+      Class.forName("datadog.context.ContextManager").getMethod("allowTesting").invoke(null);
+      Class.forName("datadog.context.ContextBinder").getMethod("allowTesting").invoke(null);
+    } catch (Throwable ignore) {
+      // don't block testing if context types aren't available
+    }
+  }
+
   @BeforeEach
   public void init() {
     // If this fails, it's likely the result of another test loading Config before it can be

dd-java-agent/instrumentation/java/java-lang/java-lang-21.0/src/main/java/datadog/trace/instrumentation/java/lang/jdk21/VirtualThreadInstrumentation.java

@@ -106,7 +106,7 @@ public static final class Construct {
     public static void afterInit(@Advice.This Object virtualThread) {
       Context context = current();
       if (context == root()) {
-        return; // no active context to propagate
+        return; // No active context to propagate, avoid creating state
       }
       VirtualThreadState state = new VirtualThreadState(context, captureActiveSpan());
       ContextStore<Object, Object> store =