Fallback to md5sum when uname fails

[AleksZyro]

Refs #16.

checksum() currently calls uname before trying the checksum command. On servers where opening a shell session for uname fails, this prevents md5sum from being attempted even when it is available.

This falls back to the Linux md5sum path when uname cannot be executed.

Regression tests exercise the real _get_system path for both ChannelOpenError and ProcessError, check quoting of paths containing spaces, and verify that a failure of the fallback command still propagates.

Validation on Linux (WSL) / Python 3.11.17:

• python -m pytest -q: 48 passed.
• black --check tests/test_sshfs.py: passed (26.1.0).
• isort --check-only tests/test_sshfs.py: passed (5.10.1).
• git diff --check: passed.

The five focused fallback cases also pass on Windows. The full Windows run has seven failures, all reproduced on the previous PR head; the project CI targets Linux and macOS.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.21%. Comparing base (15aac74) to head (2cab253).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   94.30%   94.21%   -0.10%     
==========================================
  Files          13       13              
  Lines         738      882     +144     
==========================================
+ Hits          696      831     +135     
- Misses         42       51       +9     

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[Copilot]

Pull request overview

Adds a Linux md5sum fallback when remote system detection fails.

Changes:

• Handles ChannelOpenError and ProcessError from uname.
• Adds regression coverage for session failure fallback.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
sshfs/spec.py	Implements checksum fallback logic.
tests/test_sshfs.py	Tests fallback to md5sum.

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[AleksZyro]

Hi! I wanted to follow up and ask whether this PR is still useful and whether the fallback behaviour matches the project’s expectations. I’m happy to address any requested changes or update the tests if needed. Thanks for taking a look.