Use ClassVar for Packet.fields_desc type hint

[T3pp31]

Summary

• import ClassVar in scapy.packet
• annotate Packet.fields_desc as ClassVar[List[AnyField]]
• keep runtime behavior unchanged

Closes #4991

Checks

• python3 -m py_compile scapy/packet.py
• git diff --check

Not run locally:

• tox -e flake8 (tox resolves to a broken pyenv-win shim; python3 -m tox is not installed)
• tox -e mypy (same tox issue)
• python3 -m ruff check --select RUF012 scapy/packet.py (ruff is not installed)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.29%. Comparing base (66ef96a) to head (505acab).
⚠️ Report is 30 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4998      +/-   ##
==========================================
- Coverage   80.31%   80.29%   -0.03%     
==========================================
  Files         381      383       +2     
  Lines       93630    95159    +1529     
==========================================
+ Hits        75202    76405    +1203     
- Misses      18428    18754     +326     

Files with missing lines	Coverage Δ
scapy/packet.py	84.70% <100.00%> (-0.14%)	⬇️

... and 48 files with indirect coverage changes

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[T3pp31]

CI note: after adding the required AI-Assisted commit trailer, the commit validity check is passing.

The remaining failing check is ubuntu-latest pypy3.11 root -K scanner. Its log shows TypeError: test_am() missing 1 required positional argument: 'mock_sniff' while running on PyPy 7.3.22. This appears unrelated to this PR's scapy/packet.py type-comment-only change and looks like the known PyPy 7.3.22 regression tracked in #4995.

Relevant checks for this change are passing, including Code health check, Type hints check, CodeQL, and Codacy Static Code Analysis.