fix(agent): prevent memory/swap uint64 underflow causing unrealistic usage spikes

[svenvg93]

📃 Description

Fix uint64 integer underflow in memory and swap stat collection that could cause anomalous spikes (millions of TB) in reported memory usage. When /proc/meminfo values are transiently inconsistent, gopsutil can return a Used value greater than Total, or Shared can exceed Cached + Buffers, both of which cause unsigned integer wraparound to near-MaxUint64.

fixes #1975

🪵 Changelog

🔧 Fixed

• Skip memory sample entirely when v.Used > v.Total to prevent transient /proc/meminfo inconsistencies from producing unrealistic memory usage spikes
• Fix uint64 underflow in cacheBuff calculation (Cached + Buffers - Shared) which could wrap to ~18 exabytes when Shared > Cached + Buffers; also removed the dead <= 0 guard that could never trigger on a uint64
• Fix uint64 underflow in SwapUsed calculation by guarding the subtraction before performing it