[Tests] Add unit tests for MoonshotProvider with mocked client

[Salazareo]

MoonshotProvider.ts has MoonshotProvider.integration.test.ts plus imageHandling.test.ts, but no provider-level unit test. Add a separate offline MoonshotProvider.test.ts covering model resolution, OpenAI-compat quirks, tool-call passthrough, streaming deltas, error mapping, and metering. Integration test stays untouched.

[Salazareo]

done