Cleanup of Packet::createFirstLayer

[Dimi1010]

• Changes the if-else chain into a switch case for better readability.
• Removes the duplicated new PayloadLayer(...) calls. If all validations for a link layer fail, it exits the case statement and the payload layer is created by the tail call. IMO, that is easier to read than the multiline ternary operator + static casts calls.

[codecov]

Codecov Report

❌ Patch coverage is 72.22222% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.42%. Comparing base (eb6f909) to head (5c6d14d).
⚠️ Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
Packet++/src/Packet.cpp	68.75%	9 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1981      +/-   ##
==========================================
+ Coverage   83.41%   83.42%   +0.01%     
==========================================
  Files         311      311              
  Lines       55002    54984      -18     
  Branches    11764    11752      -12     
==========================================
- Hits        45880    45873       -7     
- Misses       7842     7858      +16     
+ Partials     1280     1253      -27     

Flag	Coverage Δ
alpine320	75.92% <71.42%> (+0.01%)	⬆️
fedora42	75.85% <71.42%> (+<0.01%)	⬆️
macos-14	81.55% <31.03%> (-0.02%)	⬇️
macos-15	81.56% <31.03%> (-0.03%)	⬇️
mingw32	70.64% <65.00%> (+0.03%)	⬆️
mingw64	70.62% <65.00%> (+0.17%)	⬆️
npcap	?
rhel94	75.89% <71.42%> (+<0.01%)	⬆️
ubuntu2004	60.17% <66.66%> (-0.01%)	⬇️
ubuntu2004-zstd	60.27% <66.66%> (+0.01%)	⬆️
ubuntu2204	75.84% <71.42%> (+<0.01%)	⬆️
ubuntu2204-icpx	60.61% <3.44%> (-0.02%)	⬇️
ubuntu2404	75.93% <71.42%> (+0.01%)	⬆️
ubuntu2404-arm64	75.58% <71.42%> (-0.02%)	⬇️
unittest	83.42% <72.22%> (+0.01%)	⬆️
windows-2022	85.46% <74.07%> (+0.19%)	⬆️
windows-2025	85.49% <74.07%> (+0.14%)	⬆️
winpcap	85.49% <74.07%> (-0.06%)	⬇️
xdp	53.56% <42.85%> (+<0.01%)	⬆️

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