fix: fix connection drop with aes-gcm ciphers

[vicajilau]

Fixes #168.

This PR resolves a connection drop issue when negotiating AEAD ciphers (AES-GCM) due to an incorrect padding length check. Standard ciphers include the 4-byte packet length inside the encrypted payload boundary, whereas AEAD ciphers send it unencrypted as AAD. The padding validation is updated to use the correct AEAD-specific alignment verification.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.90%. Comparing base (bf72e2a) to head (9c31220).

Files with missing lines	Patch %	Lines
lib/src/ssh_transport.dart	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
- Coverage   53.99%   53.90%   -0.09%     
==========================================
  Files          64       64              
  Lines        5269     5272       +3     
==========================================
- Hits         2845     2842       -3     
- Misses       2424     2430       +6     

Flag	Coverage Δ
unittests	53.90% <50.00%> (-0.09%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
lib/src/ssh_transport.dart	50.59% <50.00%> (-0.78%)	⬇️

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