feat(sftp): prefer posix-rename@openssh.com extension when available

[GT-610]

Summary

The SFTP rename method now prefers the posix-rename@openssh.com extension (version 1) when advertised by the server. This performs an atomic rename with POSIX semantics (replace destination if it exists), matching the behavior of OpenSSH's sftp client.

If the server does not advertise the extension, or advertises an incompatible version, the method transparently falls back to the standard SSH_FXP_RENAME request, preserving existing behavior.

Changes

• Add SftpPosixRenameRequest extended request type in sftp_packet_ext.dart.
• Rewrite SftpClient.rename to probe the handshake extensions and use the extended request when available, falling back to SSH_FXP_RENAME on SftpExtensionError.
• Add protocol-level tests for both the extension path and the fallback path.

Test plan

• test/src/sftp/sftp_client_protocol_test.dart — all 12 tests pass (including 2 new).
• Verified fallback test exercises SSH_FXP_RENAME when no extension advertised.
• Verified extension test sends SftpExtendedPacket when extension advertised.

Backport of ServerBox's forked dartssh2.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 53.77%. Comparing base (ba899d8) to head (80b92c6).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/src/sftp/sftp_client.dart	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   53.33%   53.77%   +0.43%     
==========================================
  Files          64       64              
  Lines        5186     5198      +12     
==========================================
+ Hits         2766     2795      +29     
+ Misses       2420     2403      -17     

Flag	Coverage Δ
unittests	53.77% <91.66%> (+0.43%)	⬆️

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

Files with missing lines	Coverage Δ
lib/src/sftp/sftp_packet_ext.dart	100.00% <100.00%> (ø)
lib/src/sftp/sftp_client.dart	56.64% <87.50%> (+5.56%)	⬆️

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