Add a runtime_validator callback to generate_title() / auto_title_session()
/ maybe_auto_title(). Callers snapshot the session's model+provider when
spawning the background titler; the validator runs right before the LLM
request and skips it silently when the live runtime no longer matches —
so a stale title request can't reload a model that strict_single_load
already evicted after a user model switch. Fail-open: a raising validator
never disables titling.
Wired at all four call sites (cli, gateway, tui_gateway, acp_adapter).
Surgical reapply of PR #19137 (base was 8k+ commits stale; the original
patch predates the pinned-language prompts, the atomic-write helper, and
the moved TUI/ACP call sites). Original work by @Thatgfsj. Closes#19027.
ef1c622105