fix: resolve mypy no-any-return in _register_updated_task_definition

Annotate new_arn as Optional[str] to satisfy mypy's no-any-return check
on the dict.get() chain from boto3 response.

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

Author: singledigit

samcli/lib/sync/flows/ecs_container_sync_flow.py

@@ -221,7 +221,7 @@ def _register_updated_task_definition(self, image_uri: str) -> Optional[str]:
             LOG.warning("%sFailed to register new task definition revision", self.log_prefix, exc_info=True)
             return None
 
-        new_arn = reg_response.get("taskDefinition", {}).get("taskDefinitionArn")
+        new_arn: Optional[str] = reg_response.get("taskDefinition", {}).get("taskDefinitionArn")
         LOG.info("%sRegistered new task definition revision: %s", self.log_prefix, new_arn)
         return new_arn