Simplify sandbox worker activity tracking

Record activity names as registered and leave normalization/deduplication to the existing registration boundary.

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

Author: YunchuWang

durabletask-azuremanaged/durabletask/azuremanaged/preview/on_demand_sandbox/worker.py

@@ -68,8 +68,7 @@ def __init__(self):
 
     def add_activity(self, fn):
         activity_name = super().add_activity(fn)
-        self._on_demand_sandbox_activity_names = resolve_activity_names(
-            [*self._on_demand_sandbox_activity_names, activity_name])
+        self._on_demand_sandbox_activity_names.append(activity_name)
         return activity_name
 
     def start(self) -> None: