When gateway_routing is absent, _formatGatewayModelLabel returns '' and
the fallback path returns raw s.model — which may contain an @provider:
prefix (e.g. @token-plan-cn.xiaomimimo.com:mimo-v2.5-pro).
getModelLabel() already handles stripping @provider: prefixes (ui.js:2855)
but was never called in this fallback path. Route through it so the
session list always displays a clean model name.
Closes#3172
1a991041f1
