//! `provider/model` pair parsing, shared by every non-interactive switch
//! surface (#461 family: CLI set-model, direct-arg /models on channels and
//! the TUI). The FIRST slash splits, so model names that contain slashes
//! survive intact (`openrouter/tencent/hy3:free` → `openrouter` +
//! `tencent/hy3:free`).