07 — ModelClient、WebSocket 预连接与重试 ​

//! Session- and turn-scoped helpers for talking to model provider APIs.
//!
//! `ModelClient` is intended to live for the lifetime of a Codex session and holds the stable
//! configuration and state needed to talk to a provider (auth, provider selection, conversation id,
//! and transport fallback state).
//!
//! Per-turn settings (model selection, reasoning controls, telemetry context, and turn metadata)
//! are passed explicitly to streaming and unary methods so that the turn lifetime is visible at the
//! call site.
//!
//! A [`ModelClientSession`] is created per turn and is used to stream one or more Responses API
//! requests during that turn. It caches a Responses WebSocket connection (opened lazily) and stores
//! per-turn state such as the `x-codex-turn-state` token used for sticky routing.
//!
//! WebSocket prewarm is a v2-only `response.create` with `generate=false`; it waits for completion
//! so the next request can reuse the same connection and `previous_response_id`.
//!
//! Turn execution performs prewarm as a best-effort step before the first stream request so the
//! subsequent request can reuse the same connection.
//!
//! ## Retry-Budget Tradeoff
//!
//! WebSocket prewarm is treated as the first websocket connection attempt for a turn. If it
//! fails, normal stream retry/fallback logic handles recovery on the same turn.