fix(providers): fail loudly on malformed chat completion responses (#267) - #276

Summary of Changes

Addresses #267 by validating the chat completion response structure in OpenAICompatibleModelProvider.generate():

1. JSON parsing guard: Parses response body text safely and catches parse errors, throwing a descriptive error with HTTP status and body excerpt.
2. Response structure validation: Validates that the payload is an object and contains a non-empty choices array.
3. Choice message validation: Validates that choices[0] is an object containing string message.content.
4. Unit test coverage: Adds unit tests verifying rejections on non-JSON response body with HTTP status/excerpt context, empty choices array, missing choices, and missing/non-string message content. All existing tests continue to pass.

Closes #267