Fixes issue where LiteLLM used the json_tool_call workaround for all Groq
models, even those that support native json_schema (e.g., gpt-oss-120b,
llama-4, kimi-k2). This caused errors when users passed their own tools
alongside structured outputs.
Changes:
- Check `supports_response_schema()` before using the workaround
- Only use json_tool_call workaround for models without native support
- Add clear error message when using workaround with user-provided tools
- Update model config to set `supports_response_schema: false` for models
that don't support native json_schema
- Add unit tests for structured output handling
