Resolve sandbox transport code quality comments

Replace no-op Protocol method ellipses with explicit NotImplementedError bodies so code quality analysis does not flag the declarations.

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

Author: YunchuWang

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

@@ -19,17 +19,17 @@ class _OnDemandSandboxActivitiesStub(Protocol):
     def DeclareOnDemandSandboxActivities(
             self,
             request: pb.OnDemandSandboxActivityDeclaration) -> pb.OnDemandSandboxActivityDeclarationResult:
-        ...
+        raise NotImplementedError
 
     def RemoveOnDemandSandboxActivityDeclaration(
             self,
             request: pb.RemoveOnDemandSandboxActivityDeclarationRequest) -> pb.RemoveOnDemandSandboxActivityDeclarationResult:
-        ...
+        raise NotImplementedError
 
     def ConnectOnDemandSandboxActivityWorker(
             self,
             request_iterator: Iterable[pb.OnDemandSandboxActivityWorkerMessage]) -> pb.OnDemandSandboxActivityWorkerSessionResult:
-        ...
+        raise NotImplementedError
 
 
 class OnDemandSandboxActivitiesGrpcTransport: