fix(a2asrv): skip TaskID mismatch check when message has no task reference (fixes #350)

When loadExecutionContext receives a message without a TaskID
(e.g. on retry in cluster mode), the condition message.TaskID != tid
falsely rejects it because the empty-string TaskID cannot match any
real tid. Add an empty-string guard so messages with no task reference
are accepted.

Author: kuangmi

a2asrv/agentexec.go

@@ -192,7 +192,7 @@ func (f *factory) loadExecutionContext(ctx context.Context, tid a2a.TaskID, para
 	}
 
 	storedTask, lastVersion := taskStoreTask.Task, taskStoreTask.Version
-	if message.TaskID != tid {
+	if message.TaskID != "" && message.TaskID != tid {
 		return nil, fmt.Errorf("bug: message task id different from executor task id")
 	}