Manual title regeneration (POST /api/session/title/regenerate) and background
aux title generation failed with 422 / llm_error_aux when
auxiliary.title_generation.model in config.yaml used the WebUI-internal
@provider:model picker format (e.g. @gemini:gemini-3.1-flash-lite) — the raw
@-qualified id was forwarded to the provider API verbatim. Normalize it through
the canonical _split_webui_provider_model_value() helper before the aux call.
Co-authored-by: pamnard <pamnard@users.noreply.github.com>