Revive the FileTransporter unit tests

[tas50]

Restores unit coverage for FileTransporter — 573 lines that build every PowerShell script the upload path sends and parse every CSV report that comes back, and which has had no unit tests since June 2016.

Commit 4b9a646 ("delete file_transporter unit tests") removed an 828-line spec and nothing replaced it. That left the entire unit suite at three examples against TmpZip, finishing in 15ms, while the code that actually talks to Windows went untested.

Unit examples go from 3 to 29.

Why this is a rewrite, not a revert

Both the spec and the code moved on since 2016, so git revert was never an option:

2016	today
MD5 digests	Digest::SHA1
Base64 temp files + decode_files	streaming into a FileStream + extract_files
o[:exitcode] = 10	WinRM::Output#exitcode= and #<<

What is covered

• the rendered check_files and extract_files PowerShell hash tables, including ps_hash formatting
• streamed file contents, asserted by decoding every Base64 payload and comparing to the source bytes
• directory uploads via a temporary zip, the extract_files round trip, and cleanup of the temp zip
• StringIO uploads and the single-source restriction (UploadSourceError)
• destination reconciliation when the remote target turns out to be a folder
• Base64 transfer-size accounting and the progress callback
• FileTransporterFailed on a non-zero exit code, and on stderr with a zero exit code
• Errno::ENOENT for a missing local path

Two choices that should keep these from rotting again

The shell is an instance_double(WinRM::Shells::Powershell), not a plain double. A plain double is what let the old specs drift — it happily accepts calls to methods the real class no longer has. With a verifying double, a rename in chef-winrm fails the suite instead of passing against a stale stub.

The fake host answers check_files using whichever digests the transporter actually asked for, read back out of the rendered script. Zip digests change on every run, so this keeps the directory tests honest without pinning them to a value that cannot be predicted.

Streaming is likewise asserted on decoded bytes rather than an expected chunk count, so the tests survive changes to PSRP fragment sizing.

Verification

Mutation-tested against the implementation — each of these fails the suite:

Mutation	Failures
Digest::SHA1 → Digest::MD5	10
ps_hash separator ";\n" → ",\n"	2
drop zip_io.unlink in cleanup	1
disable target_is_folder reconciliation	1
invert the chk_dirty check	7
size / 3 * 4 → size	2
always skip extract_files	2
drop the stderr check in parse_response	1
remove the digest from the tmpzip name	1

bundle exec rake spec → 29 examples, 0 failures. cookstyle --chefstyle -c .rubocop.yml → 17 files, no offenses.

Note

No production code is touched — this PR is spec-only.

Local verification on Ruby 4.0 needs the logger dependency that is currently only on the ci/test-all-supported-rubies branch, since lib/chef-winrm-fs.rb requires logger and it stopped being a default gem in Ruby 4.0. This does not affect CI on main, whose matrix is Ruby 3.1 and 3.4.