feat(sftp): add downloadToRandomAccess for offset-based download

[GT-610]

Summary

Adds SftpFile.downloadToRandomAccess, which downloads a file into a dart:io RandomAccessFile, writing each SFTP read reply at its own offset rather than requiring replies to arrive in order.

Unlike downloadTo (which streams sequentially into a StreamSink), downloadToRandomAccess uses a completion queue that decouples the order in which pipelined read replies arrive from the order in which they are written to disk. This lets pipelined reads keep making progress when a later offset completes before an earlier one, improving throughput on high-latency or reordering links.

Chain note

This PR is built on top of #172 (feat(sftp): prefer posix-rename@openssh.com). If #172 is merged first, only the downloadToRandomAccess commits will show in this PR's diff. If #172 is still open, both features are included here — please review and merge #172 first, then this PR will auto-rebase to show only the remaining changes.

Changes

• Add SftpFile.downloadToRandomAccess(RandomAccessFile destination, ...):
  • Adaptive concurrency: starts with one outstanding read and ramps up to maxPendingRequests as replies succeed, backing off the chunk size on short reads to avoid over-requesting past EOF.
  • Validates chunkSize and maxPendingRequests as positive.
  • Throws SftpError on incomplete downloads (byte count mismatch).
• Add a private _ReadCompletion helper class (kept SDK 2.17-compatible — no Dart 3 record syntax).
• Add a protocol-level test that issues reads in pipelined fashion and replies out of order, asserting the final file contents are written at the correct offsets.

Test plan

• test/src/sftp/sftp_client_protocol_test.dart — all 13 tests pass (including the new out-of-order test).
• Verified the test replies read3 before read2 and the output file is still in offset order.
• Ran dart analyze — no new warnings introduced.

Backport of ServerBox's forked dartssh2.

[codecov]

Codecov Report

❌ Patch coverage is 67.60563% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.99%. Comparing base (5c77884) to head (41ed8e1).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/src/sftp/sftp_client.dart	67.60%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
+ Coverage   53.84%   53.99%   +0.14%     
==========================================
  Files          64       64              
  Lines        5198     5269      +71     
==========================================
+ Hits         2799     2845      +46     
- Misses       2399     2424      +25     

Flag	Coverage Δ
unittests	53.99% <67.60%> (+0.14%)	⬆️

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_client.dart	58.38% <67.60%> (+1.20%)	⬆️

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