Add a persisted, backend-confirmed provider/model lock for Hermes
Browser and other session API clients. A confirmed lock is an
execution contract rather than response metadata:
- POST /api/sessions/{session_id}/model validates and persists a
confirmed browser_model_lock (advertised in /v1/capabilities)
- session chat + chat/stream consume the persisted lock on body-only
follow-up turns; a confirmed lock wins over an older gateway session
/model override and the session-persisted model
- a later successful session /model switch explicitly clears and
replaces the lock while preserving lineage markers (_branched_from)
and invalidating cached system-prompt model/provider metadata
- ordinary one-off request overrides never replace a confirmed lock
- provider-resolution failure fails closed as a typed provider-auth
error (controlled response, never global-credential reuse)
- confirmed locks disable the global fallback model chain
- the completed agent's actual provider/model must match the locked
route or the turn fails with a runtime-mismatch error
- responses carry sanitized runtime metadata reporting actual vs
requested provider/model and lock state
Rebased onto the provider-aware request routing (#70853) and
session-model parity (#70931) that landed since the original branch;
the lock now slots into that precedence chain as the top rung.
Salvaged from PR #61236 by @abundantbeing.
7cd48733db