Harden ClassAd parsing in prepare-env (anchored reads, defaults, memory units)

[oshadura]

Summary

The HTCondor ClassAd parsing in docker/prepare-env/prepare-env-cc-analysis.sh
works, but it's fragile. It extracts attributes with grep <attr> | awk '{print $NF}',
has no fallbacks when an attribute is missing, and formats the memory limit with a
string hack ($MEMORY".00MB"). This is the "careful parsing going forward" cleanup
flagged back in #8 — the functional part (cores/memory/name are forwarded to
dask-worker) already landed; this issue is purely about hardening it.

Why

• Fragile extraction. grep "DaskWorkerName " (trailing space) and awk '{print $NF}'
  rely on whitespace/field positioning and trailing-space tricks to avoid prefix
  collisions. A line-anchored read of Attr = value is sturdier.
• No empty-value handling. If an attribute is missing, the script emits a broken
  flag (e.g. --nthreads with no value) and the worker dies with a cryptic error
  instead of a clear message.
• Memory unit hack. MEMORY_MB_FORMATTED=$MEMORY".00MB" assumes RequestMemory
  is always a clean integer MB value and concatenates a unit string. The attribute
  intended for this is DaskWorkerMemory (bytes, set by dask-jobqueue), which
  dask-worker --memory-limit accepts directly.

Proposed changes

• Replace grep | awk attribute extraction with an anchored helper that reads
  ^<attr> = <value> from $_CONDOR_JOB_AD.
• Add sane defaults for non-critical attributes (cores, name) and fail-fast guards
  for the networking attributes the worker can't start without (ports, host).
• Prefer DaskWorkerMemory (bytes) for --memory-limit, falling back to
  RequestMemory (MB) only if it's absent; drop the .00MB string formatting.
• Verify on a real cluster that a worker requesting non-default cores/RAM comes up
  with the matching --nthreads / --memory-limit.

Out of scope

DaskWorkerDisk handling (pointing --local-directory at writable scratch) is related
but tracked separately in #421.

References

• Original parsing discussion: [coffea_casa] Parse Dask worker arguments in prepare-env.sh #8
• File: docker/prepare-env/prepare-env-cc-analysis.sh