Add flush() method to SSHSocket, SSHClient, and SSHChannel

[vicajilau]

This pull request introduces a new flush() method to the SSHSocket, SSHClient, and SSHChannel classes, allowing users to force the flushing of any buffered outgoing data. The implementation is propagated across the main library code, channel controllers, and test fakes to ensure consistent behavior and testability.

New flush functionality

• Added a flush() method to the SSHSocket abstract class and implemented it in all concrete socket classes, including _SSHNativeSocket and SSHForwardChannel, to force flushing of buffered outgoing data.
• Implemented flush() in SSHTransport to call the underlying socket's flush() method, ensuring transport-level flushing.
• Added a flush() method to SSHClient that delegates to the transport layer, and passed this method into SSHChannelController for channel-level flushing.
• Exposed a flush() method on SSHChannel that delegates to its controller, and updated SSHChannelController to support an optional onFlush callback and provide a flush() method.

Test support

• Updated all relevant test fake socket classes to implement a no-op flush() method, ensuring compatibility with the new interface in tests.

Documentation

• Updated CHANGELOG.md to document the addition of the flush() method to the public API.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.88%. Comparing base (f8cac95) to head (f83aae1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #185      +/-   ##
==========================================
+ Coverage   55.42%   55.88%   +0.45%     
==========================================
  Files          66       66              
  Lines        5452     5470      +18     
==========================================
+ Hits         3022     3057      +35     
+ Misses       2430     2413      -17     

Flag	Coverage Δ
unittests	55.88% <100.00%> (+0.45%)	⬆️

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

Files with missing lines	Coverage Δ
lib/src/socket/ssh_socket.dart	100.00% <100.00%> (+100.00%)	⬆️
lib/src/socket/ssh_socket_io.dart	42.10% <100.00%> (+42.10%)	⬆️
lib/src/ssh_channel.dart	50.00% <100.00%> (+0.67%)	⬆️
lib/src/ssh_client.dart	24.56% <100.00%> (+1.61%)	⬆️
lib/src/ssh_forward.dart	76.19% <100.00%> (+3.96%)	⬆️
lib/src/ssh_session.dart	70.12% <100.00%> (+1.63%)	⬆️
lib/src/ssh_transport.dart	52.74% <100.00%> (+0.15%)	⬆️

... and 1 file with indirect coverage changes

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