bfd: fix ConditionalField condition for optional_auth always being True

[T3pp31]

Summary

Fixes #4937

• The ConditionalField guarding optional_auth in BFD used pkt.flags.names[2] == "A", which always evaluates to True because FlagValue.names returns the flag definition string "MDACFP", not the set of currently active flags
• Replaced with pkt.flags.A to properly check the Authentication Present bit, consistent with the pattern used elsewhere in Scapy (e.g., vxlan.py)
• Added 4 regression tests covering: no-auth dissection, non-auth flags, round-trip serialization, and auth-present dissection

Test plan

• All 12 BFD tests pass (8 existing + 4 new)
• No regression in existing auth dissection and build tests
• CI passes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.31%. Comparing base (9ae49f8) to head (0d8a85a).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4965   +/-   ##
=======================================
  Coverage   80.30%   80.31%           
=======================================
  Files         379      379           
  Lines       93164    93181   +17     
=======================================
+ Hits        74820    74840   +20     
+ Misses      18344    18341    -3     

Files with missing lines	Coverage Δ
scapy/contrib/bfd.py	94.73% <ø> (ø)

... and 5 files with indirect coverage changes

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[gpotter2]

Thanks for the PR !