chore: add logging to TaskManager (#205)

# Description

I noticed while reviewing #181 and looking at the Python implementation
that we don't have logging in the TaskManager. We decided on the logging
strategy after the initial code was written. This PR addresses this
omission.

Author: kabir

sdk-server-common/src/main/java/io/a2a/server/tasks/TaskManager.java

@@ -16,8 +16,13 @@
 import io.a2a.spec.TaskArtifactUpdateEvent;
 import io.a2a.spec.TaskStatus;
 import io.a2a.spec.TaskStatusUpdateEvent;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 public class TaskManager {
+
+    private static final Logger LOGGER = LoggerFactory.getLogger(TaskManager.class);
+
     private volatile String taskId;
     private volatile String contextId;
     private final TaskStore taskStore;
@@ -101,16 +106,18 @@ Task saveTaskEvent(TaskArtifactUpdateEvent event) throws A2AServerException {
             // This represents the first chunk for this artifact index
             if (existingArtifactIndex >= 0) {
                 // Replace the existing artifact entirely with the new artifact
+                LOGGER.debug("Replacing artifact at id {} for task {}", artifactId, taskId);
                 artifacts.set(existingArtifactIndex, newArtifact);
             } else {
                 // Append the new artifact since no artifact with this id/index exists yet
+                LOGGER.debug("Adding artifact at id {} for task {}", artifactId, taskId);
                 artifacts.add(newArtifact);
             }
 
         } else if (existingArtifact != null) {
             // Append new parts to the existing artifact's parts list
             // Do this to a copy
-
+            LOGGER.debug("Appending parts to artifact id {} for task {}", artifactId, taskId);
             List<Part<?>> parts = new ArrayList<>(existingArtifact.parts());
             parts.addAll(newArtifact.parts());
             Artifact updated = new Artifact.Builder(existingArtifact)
@@ -120,6 +127,9 @@ Task saveTaskEvent(TaskArtifactUpdateEvent event) throws A2AServerException {
         } else {
             // We received a chunk to append, but we don't have an existing artifact.
             // We will ignore this chunk
+            LOGGER.warn(
+                    "Received append=true for nonexistent artifact index for artifact {} in task {}. Ignoring chunk.",
+                    artifactId, taskId);
         }
 
         task = new Task.Builder(task)