feat: add SSHSession waitForExit timeout

[GT-610]

Backport of ServerBox's fork.

Summary

Adds SSHSession.waitForExit({Duration? timeout}).

The method completes when the session receives an exit-status or exit-signal channel request. It also completes with the current exit code when the channel closes without an explicit exit request. If a timeout is provided and elapses first, it returns null without closing the session.

Test plan

• dart test test/src/ssh_client_run_with_result_test.dart
• dart analyze lib/src/ssh_session.dart test/src/ssh_client_run_with_result_test.dart

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.61%. Comparing base (43a8980) to head (f4b3e9e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
+ Coverage   54.52%   54.61%   +0.08%     
==========================================
  Files          66       66              
  Lines        5357     5367      +10     
==========================================
+ Hits         2921     2931      +10     
  Misses       2436     2436              

Flag	Coverage Δ
unittests	54.61% <100.00%> (+0.08%)	⬆️

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Files with missing lines	Coverage Δ
lib/src/ssh_session.dart	68.49% <100.00%> (+5.00%)	⬆️

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