Include instructions param in OpenAILanguageModel

[twocentstudios]

This PR updates OpenAILanguageModel to include the LanguageModelSession.instructions parameter in requests when provided.

• For the Responses API, instructions is appended to the body as instructions (docs).
• For the Chat Completions API, instructions is appended to the messages array with the system tag. (Note: the docs say that developer is preferred over system, but it's presumably backwards compatible).

If disregarding instructions was the intended behavior, feel free to close this PR.

[mattt]

Thanks, @twocentstudios! This was indeed an oversight. I appreciate your contributing this fix.

[mattt]

Thanks again for your help, @twocentstudios. This is now available in 0.3.2.

[twocentstudios]

Thanks! For the quick version release and for this library and for Sosumi 🙏