[codex] Round-trip model-bound turn state

[aibrahim-oai]

What

• bind the opaque x-codex-turn-state token to the requested model slug
• round-trip request-scoped turn state through HTTP, WebSocket response.create.client_metadata, and compact responses
• explicitly reset WebSocket turn state for a new turn/model while preserving handshake-only servers as a deferred fallback
• reuse the same model-bound client session across inline compaction

Companion to backend PR: https://github.com/openai/openai/pull/1018026

Why

WebSocket upgrade headers are connection-scoped, so a reused connection cannot carry the correct per-request turn state. Compact responses also previously dropped the response header. A single unqualified OnceLock could retain state from an older requested model during pre-turn compaction.

The client now treats response metadata as authoritative, uses a handshake token only after a response attempt produces no metadata, and replaces the state when the requested slug changes.

Validation

• cargo check -p codex-core
• focused codex-core routing/WS/compact suite: 8 passed
• just test -p codex-api: 123 passed
• just fix -p codex-api
• cargo fmt --all

[aibrahim-oai]

Superseded by the smaller stacked PRs #27930 → #27931 → #27929. The new stack removes the handshake fallback and keeps HTTP, compact, and WebSocket request metadata reviewable independently.