feat: add MiniMax M3 model - #191

Reason: Add the MiniMax-M3 model so it can be selected with its one-million-token context configuration.

This adds MiniMax-M3 to the model registry with the target context limit and exposes it in the OpenAI-compatible model selector.

Checks:

• git diff --check
• Dependency-backed lint was not run because the repository dependencies are not installed in the isolated worktree.