Fix Packet.fields_desc type annotation to match runtime behavior

[Ts-Boom]

Description

Packet.fields_desc was typed as ClassVar[List[AnyField]], but at
runtime Packet_metaclass.__new__ explicitly handles the case where
elements can be Packet_metaclass instances (references to another
packet's fields_desc):

# scapy/base_classes.py ~line 372
current_fld = dct["fields_desc"]
# type: List[Union[scapy.fields.Field[Any, Any], Packet_metaclass]]

This means the annotation was incorrect for the pre-resolution state of the list, causing type checkers to flag valid usage like:

fields_desc = [AnotherPacket, ByteField('x', 0)]

Fix

Changed the annotation from List[AnyField] to List[Union[AnyField, Packet_metaclass]].
Packet_metaclass is already imported in packet.py (line 44) and Union is already in the typing imports — no new dependencies added.

References

• Fixes Packet.fields_desc type annotation is inconsistent with runtime behavior #5018

AI Disclosure

No AI tools were used.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.36%. Comparing base (94a853b) to head (ac66137).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5187      +/-   ##
==========================================
- Coverage   80.37%   80.36%   -0.01%     
==========================================
  Files         375      375              
  Lines       97675    97675              
==========================================
- Hits        78506    78501       -5     
- Misses      19169    19174       +5     

Files with missing lines	Coverage Δ
scapy/packet.py	84.79% <100.00%> (ø)

... and 7 files with indirect coverage changes

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