Fix fields_desc type hint to allow Packet class references

[T3pp31]

Summary

• widen Packet.fields_desc type annotation to ClassVar[List[Union[AnyField, Type[Packet]]]]
• add _resolved_fields_desc() helper to cast resolved field lists for mypy at runtime usage sites
• no change to build/dissect behavior

Closes #5018

Why _resolved_fields_desc() was added

The type annotation alone is sufficient to fix #5018, but widening fields_desc to
List[Union[AnyField, Type[Packet]]] causes mypy to treat runtime iteration over
self.fields_desc as possibly containing Type[Packet]. That breaks CI (tox -e mypy
on Python 3.12): 22 new errors in scapy/packet.py with the one-line change alone.

At runtime, Packet_metaclass.__new__ already resolves Packet references to Field
instances before any instance method runs. _resolved_fields_desc() is therefore a
typing-only helper (cast + list) with no behavioral change; it documents that
call sites operate on the post-metaclass, field-only list.

Why no new UTScapy tests

Per CONTRIBUTING.md, this is a typing-only fix with no runtime behavior change:

• build/dissect/show paths are unchanged
• the Packet-in-fields_desc pattern is already exercised by existing tests, e.g.
  test/contrib/loraphy2wan.uts (LinkADRReq inlines DataRate_TXPower and
  Redundancy Packet classes in fields_desc)
• regression coverage for this change is provided by tox -e mypy and tox -e flake8

Happy to add a minimal .uts test if maintainers prefer explicit coverage.

Test plan

• tox -e flake8
• tox -e mypy (Python 3.12)

Made with Cursor

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.34%. Comparing base (14cc726) to head (118ed51).

Files with missing lines	Patch %	Lines
scapy/packet.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5020   +/-   ##
=======================================
  Coverage   80.34%   80.34%           
=======================================
  Files         386      386           
  Lines       96012    96014    +2     
=======================================
+ Hits        77137    77143    +6     
+ Misses      18875    18871    -4     

Files with missing lines	Coverage Δ
scapy/packet.py	84.72% <94.73%> (+0.02%)	⬆️

... and 4 files with indirect coverage changes

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[gpotter2]

Hi & thanks for the PR.
The fix provided in the original issue looked more correct: don't add a function, just update the one typing that's incorrect and use # type: ignore if absolutely necessary elsewhere.