fix: CancelTaskAsync re-fetches task after UpdateStatusAsync (alternative approach) (#283)

* fix: re-fetch task after UpdateStatusAsync in CancelTaskAsync

CancelTaskAsync discarded the return value of ITaskStore.UpdateStatusAsync,
so the returned AgentTask reflected the pre-cancellation status. Beyond just
Status, a custom store may update additional fields during status transitions
(e.g. UpdatedAt, ETag). Re-fetching the full task after the update ensures the
returned AgentTask is fully authoritative for all store implementations.

The bug was masked by InMemoryTaskStore because GetTaskAsync returns a reference
to the cached object that UpdateStatusAsync mutates in place.

Fixes #277

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: throw instead of silently falling back to stale task on null re-fetch

If GetTaskAsync returns null after a successful UpdateStatusAsync, ?? task
would return the original object with pre-cancellation status, reintroducing
the bug. Throw instead to surface the inconsistency loudly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Gijs Walraven <gijs.walraven@delaware.pro>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Roman Konecny <rokonecn@microsoft.com>

Author: 4 people

src/A2A/Server/TaskManager.cs

@@ -109,7 +109,10 @@ public async Task<AgentTask> CreateTaskAsync(string? contextId = null, string? t
                 throw new A2AException("Task is in a terminal state and cannot be cancelled.", A2AErrorCode.TaskNotCancelable);
             }
 
-            task.Status = await _taskStore.UpdateStatusAsync(task.Id, TaskState.Canceled, cancellationToken: cancellationToken).ConfigureAwait(false);
+            await _taskStore.UpdateStatusAsync(task.Id, TaskState.Canceled, cancellationToken: cancellationToken).ConfigureAwait(false);
+            task = await _taskStore.GetTaskAsync(task.Id, cancellationToken).ConfigureAwait(false)
+                ?? throw new A2AException("Task not found after cancellation.", A2AErrorCode.TaskNotFound);
+                
             await OnTaskCancelled(task, cancellationToken).ConfigureAwait(false);
             return task;
         }