Proxmox status.Mem includes page cache as "used" memory, inflating
reported VM usage. The existing fallbacks (balloon meminfo, RRD, linked
host agent) were frequently unavailable, causing most VMs to fall
through to the inflated status-mem source.
Adds a new last-resort fallback that reads /proc/meminfo via the QEMU
guest agent file-read endpoint to get accurate MemAvailable. Results
are cached (60s positive, 5min negative backoff for unsupported VMs).
Also fixes: RRD memavailable fallback missing from traditional polling
path, cache key collisions in multi-PVE setups, FreeMem underflow
guard inconsistency, and integer overflow in kB-to-bytes conversion.
- Fix TestMonitor_PollGuestSnapshots_Coverage by correctly initializing State ID fields
- Improve PBS client to handle alternative datastore metric fields (total-space, etc.)
- Add comprehensive test coverage for PBS polling, auth failures, and datastore metrics
- Add various coverage tests for monitoring, alerts, and metadata handling
- Refactor Monitor to support better testing of client creation and auth handling
