Fix: forward client_decorator to the Benchling SDK

[edx4900]

Summary

BenchlingService subclasses the Benchling SDK's Benchling class, whose __init__ accepts a client_decorator for customizing the underlying BenchlingApiClient. But BenchlingService.__init__ defines its own narrower signature and forwards only url, auth_method, and retry_strategy to super().__init__, so the decorator is unreachable for callers.

This PR adds an optional client_decorator parameter to BenchlingService.__init__ and forwards it to the SDK. It's backwards-compatible (defaults to None).

Closes #209