fix(llm): send whip user agent

[zerone0x]

Summary

• stamp User-Agent: whip on OpenAI-compatible provider requests
• centralize provider request headers for model listing, streaming chat, and completion requests
• add coverage for model, stream, and complete requests

Fixes #75

Testing

• go test ./internal/llm

AI Disclosure

This PR was implemented by an autonomous AI coding agent and reviewed before submission.

[atbe]

thanks for the contribution @zerone0x ! Just out of curiosity, what benefit do these user agents provide?