fix(system): do not fail memory snapshots on inaccessible processes (#2078)

- `get_memory_info()` no longer raises when a process cannot be
inspected. In restricted environments (`hidepid`, hardened containers)
it failed on every sample, which meant a traceback in the log every
second and an autoscaler running on a stale memory history. Processes
that deny inspection, exit mid-measurement, or lose their `/proc` entry
are now skipped instead.
- Failure to list the child processes no longer aborts the snapshot
either - the parent's own usage is still reported.
- A kernel that exposes no `smaps` at all makes psutil alias
`memory_full_info` to `memory_info`, whose result has no `pss` field, so
reading it raised `AttributeError` on every sample. The metric is now
read defensively, and the verdict is latched: such a machine is detected
once instead of being re-probed for every process on every sample.
- A `smaps` file can be empty, which psutil parses to a PSS of zero.
That was reported as zero bytes used, which the autoscaler reads as free
memory. A PSS of zero now falls back to the RSS of the same process,
which `memory_full_info()` has already read anyway.
- A single process denying PSS falls back to RSS just for itself - a
denial says nothing about the other processes, so it is not latched.
- Every degraded path warns once, so an estimate that misses a
subprocess or falls back from PSS to RSS shows up in the log instead of
being silently wrong.
- `LoggerOnce` is now thread-safe, since memory metrics are sampled in a
worker thread - this PR adds its first caller that runs off the event
loop.
- Nothing changes when PSS is readable: on a normal Linux process tree
the reported `current_size` is byte-for-byte what `master` reports, with
no warnings and no RSS fallbacks.
- Tests cover the denied, exited, zombie, and missing-`/proc` cases,
both PSS fallbacks and their warnings, the latch, and a vanished process
not being misreported as a denial.

*✍️ Drafted by Claude Code*

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Co-authored-by: Vlada Dusek <v.dusek96@gmail.com>