feat: add SSHClient handshake and auth timeouts

[GT-610]

Summary

This is a backport from ServerBox's fork.

It adds two optional SSHClient timeout controls:

• handshakeTimeout: maximum time to wait for the transport handshake to reach ready state
• authTimeout: maximum time to wait for user authentication after the transport is ready

Both options default to null, so existing behavior is unchanged unless callers opt in.

On timeout:

• handshake timeout completes client.authenticated with SSHHandshakeError('Handshake timed out')
• auth timeout completes client.authenticated with SSHAuthAbortError('Authentication timed out')

Timers are cancelled on success, connection close, and error paths.

Test plan

• dart test test/src/ssh_client_timeout_test.dart test/src/ssh_auth_abort_error_test.dart test/src/ssh_client_ident_test.dart test/src/ssh_client_run_with_result_test.dart
• dart analyze lib/src/ssh_client.dart test/src/ssh_client_timeout_test.dart

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.34%. Comparing base (c2bf398) to head (796b42e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
+ Coverage   54.85%   55.34%   +0.49%     
==========================================
  Files          66       66              
  Lines        5423     5449      +26     
==========================================
+ Hits         2975     3016      +41     
+ Misses       2448     2433      -15     

Flag	Coverage Δ
unittests	55.34% <100.00%> (+0.49%)	⬆️

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

Files with missing lines	Coverage Δ
lib/src/ssh_client.dart	22.95% <100.00%> (+5.28%)	⬆️

... and 1 file with indirect coverage changes

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