Off-thread reachability probe for a SAVED server we are about to switch to.

A deep-link server switch tears down the current session before re-authing against the target. If the target is offline, that would strand the user — so we pre-flight it with a non-destructive GET to its public system-info endpoint BEFORE any teardown. This runs on a Task thread (raw roUrlTransfer, no Jellyfin auth) rather than the API pool because the probe targets a DIFFERENT server than the active session: the pool would leak the current session's auth token cross-server and assumes m.global.server. See docs/architecture/api.md (Pattern 4) and docs/architecture/navigation.md ("Deep links").

Non-destructive: a raw GET to the saved canonical URL's public system-info endpoint. Does NOT touch m.global.server (unlike server.UpdateURL). Returns true only on HTTP 200 + a genuine Jellyfin server response, false on any error or an 8s timeout.