[Tests] Add unit tests for ZAIProvider with mocked client

[Salazareo]

ZAIProvider.ts only has ZAIProvider.integration.test.ts. Add a separate offline ZAIProvider.test.ts covering model resolution, OpenAI-compat shape quirks, tool-call passthrough, streaming deltas, error mapping, and metering. Integration test stays untouched.

[Salazareo]

done