Explicitly don't support Responses API instructions

[bxyu-nvidia]

Use cases, pain points, and background instructions is a valid Response create parameter https://platform.openai.com/docs/api-reference/responses/create#responses_create-instructions. however, we don't have a clear implementation of this.

Description: For the moment, we should explicitly error out when instructions are provided on Response create. Eventually we may want to support this when there is clearer guidance from OpenAI on what this maps to underneath the hood e.g. system or developer message.

Design: Should just touch nemo_gym/openai_utils.py

Out of scope:

Acceptance Criteria:

• [ ] Implementation of the above
• [ ] Test

[cwing-nvidia]

classifying this as a bug: current unclear/undefined handling of valid OpenAI parameter is problematic behavior that needs fixing.

[cmunley1]

I think we could just treat as system message