feat: add SSH agent forwarding support

[vicajilau]

Summary

• integrate PR Add SSH agent forwarding support #139 (agent forwarding) on top of current master
• resolve conflicts in SSH client/channel while preserving recent fixes
• include changelog entry for Add SSH agent forwarding support #139

Details

• adds auth-agent request/channel handling with in-memory agent support
• keeps existing X11 forwarding flow and recent transport/auth updates
• includes agent tests and passes targeted regression suite

Validation

• dart test test/src/ssh_agent_test.dart test/src/ssh_transport_version_test.dart test/src/ssh_auth_abort_error_test.dart

[codecov]

Codecov Report

❌ Patch coverage is 69.93007% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.72%. Comparing base (84a2262) to head (5ef6bf9).
⚠️ Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/src/ssh_client.dart	3.70%	26 Missing ⚠️
lib/src/ssh_agent.dart	89.18%	12 Missing ⚠️
lib/src/ssh_channel.dart	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   41.45%   42.72%   +1.26%     
==========================================
  Files          60       61       +1     
  Lines        4648     4791     +143     
==========================================
+ Hits         1927     2047     +120     
- Misses       2721     2744      +23     

Flag	Coverage Δ
unittests	42.72% <69.93%> (+1.26%)	⬆️

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