Fix span lifecycle issues found in code review

- Activity worker: track error separately, end span once in finally block
  (fixes double span.end() call on error path)
- OrchestrationRunner: close scope before ending span, add null check
  (fixes scope/span lifecycle ordering and potential NPE)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: torosent

client/src/main/java/com/microsoft/durabletask/DurableTaskGrpcWorker.java

@@ -286,22 +286,22 @@ public void startAndBlock() {
                         // TODO: Run this on a worker pool thread: https://www.baeldung.com/thread-pool-java-and-guava
                         String output = null;
                         TaskFailureDetails failureDetails = null;
+                        Throwable activityError = null;
                         try {
                             output = taskActivityExecutor.execute(
                                 activityRequest.getName(),
                                 activityRequest.getInput().getValue(),
                                 activityRequest.getTaskId());
                         } catch (Throwable e) {
-                            TracingHelper.endSpan(activitySpan, e);
-                            activitySpan = null;
+                            activityError = e;
                             failureDetails = TaskFailureDetails.newBuilder()
                                 .setErrorType(e.getClass().getName())
                                 .setErrorMessage(e.getMessage())
                                 .setStackTrace(StringValue.of(FailureDetails.getFullStackTrace(e)))
                                 .build();
                         } finally {
                             activityScope.close();
-                            TracingHelper.endSpan(activitySpan, null);
+                            TracingHelper.endSpan(activitySpan, activityError);
                         }
 
                         ActivityResponse.Builder responseBuilder = ActivityResponse.newBuilder()

client/src/main/java/com/microsoft/durabletask/OrchestrationRunner.java

@@ -177,17 +177,18 @@ public TaskOrchestration create() {
 
         // TODO: Error handling
         TaskOrchestratorResult taskOrchestratorResult;
+        Throwable orchestrationError = null;
         try {
             taskOrchestratorResult = taskOrchestrationExecutor.execute(
                     orchestratorRequest.getPastEventsList(),
                     orchestratorRequest.getNewEventsList());
         } catch (Exception e) {
-            TracingHelper.endSpan(orchestrationSpan, e);
+            orchestrationError = e;
             throw e instanceof RuntimeException ? (RuntimeException) e : new RuntimeException(e);
         } finally {
-            orchestrationScope.close();
+            if (orchestrationScope != null) orchestrationScope.close();
+            TracingHelper.endSpan(orchestrationSpan, orchestrationError);
         }
-        TracingHelper.endSpan(orchestrationSpan, null);
 
         OrchestratorService.OrchestratorResponse response = OrchestratorService.OrchestratorResponse.newBuilder()
                 .setInstanceId(orchestratorRequest.getInstanceId())