Back-port v5 fix 0dca8a037 to v6. Extract pollPVEBackupsAndSnapshots:
the two backup-inventory scans share a bounded backupCtx, which is then
cancelled, and pollGuestSnapshots runs on the parent context. Because
the parent has no deadline, pollGuestSnapshots establishes its own
60s-4min budget instead of inheriting an already-exhausted backup
deadline and skipping entirely (the v6 early-return at
monitor_backups.go made this strictly worse). Adds a regression test
proving snapshots still poll after the storage scan exhausts its budget.
rcourtman