SaveFileAsync: write hidden files again, check the SaveFilesAsync folder; test on Windows - #97

Part of 3.3.0. This fixes what the review of the merged #84 found, before the change is released.

What changed

1. Hidden and system files on Windows (ConvertApi/ConvertApiExtension.cs)

• FileMode.Create (CREATE_ALWAYS) throws UnauthorizedAccessException on an existing file with the Hidden or System attribute (UnauthorizedAccessException when writing to a file with Hidden attribute dotnet/runtime#117757). 3.2.2 wrote such files.
• SaveFileAsync therefore opens the file with FileInfo.OpenWrite() again, as 3.2.2 did, and after the copy cuts it to the written length: if (fileStream.CanSeek && fileStream.Length > fileStream.Position) fileStream.SetLength(fileStream.Position);. That keeps PR 84's fix for corruption when saving over a longer file.
• The CanSeek guard is there because an unguarded SetLength throws on a device such as /dev/null.

2. CI no longer passes with zero tests. The test step passes -- RunConfiguration.TreatNoTestsAsError=true.

3. No Windows CI. A test (windows) job was tried here and passed, then dropped by decision: the tests run on Linux only, as before. Apart from the TreatNoTestsAsError flag, build.yml and the test project are unchanged.

4. The tests no longer leak static state. SaveFileTests restores ConvertApi.ApiToken and the static client in [TearDown].

5. SaveFilesAsync checks its folder argument before downloading anything (option A).

• It throws ArgumentException, naming SaveFileAsync, when the argument:
  • is blank;
  • is an existing file;
  • or is a missing path with no trailing separator whose extension matches a result file's, such as SaveFilesAsync("result.pdf") for PDF results.
• Without this check, main would quietly create a folder named result.pdf, which then blocks SaveFileAsync("result.pdf").
• "output", "", ".", "v1.2", "archive.pdf/" and an existing folder named archive.pdf all still work.
• An empty result list returns at once and creates no folder, as in 3.2.2.

6. CHANGELOG

• The wording is now "Every version since 2.0.0 (2018) has this bug."
• Under Changed:
  • a folder that cannot be created now throws before the download, for example with UnauthorizedAccessException, which is not an IOException. 3.2.2 threw DirectoryNotFoundException after downloading.
  • the new ArgumentException.

The 0.0.0-local pack still shows NU5048 (PackageIconUrl). That warning was already on main and is fixed in the 3.3.0 metadata PR.

How it was tested

• The CI steps, run locally on macOS: restore, Release build (the same 3 warnings as main), dotnet test ... -- RunConfiguration.TreatNoTestsAsError=true (17 of 17 pass), and pack.
• New unit tests:
  • saving over a Hidden + System file. It only tests something on Windows, where FileMode.Create failed; CI runs it on Linux, where it always passes;
  • saving over a shorter file;
  • saving to /dev/null (Unix only);
  • "result.pdf", "RESULT.PDF" and "nested/result.pdf" are rejected with no folder created and no download;
  • an existing file is rejected and left unchanged;
  • a blank argument is rejected;
  • "archive.pdf/", "v1.2" and "output.d" are accepted;
  • an existing archive.pdf folder is used;
  • an empty result list creates no folder.
• Against the staging API, a docx to pdf conversion with this branch:
  • SaveFileAsync over a 400,000-byte file left exactly the 146,734-byte PDF;
  • SaveFilesAsync into a missing nested folder created it and saved the file;
  • SaveFilesAsync(".../result.pdf") threw ArgumentException and created no folder;
  • DeleteFilesAsync removed both server files.
• Read the API token from CONVERTAPI_TOKEN when none is set in code #96 is merged into main, and main is merged into this branch (CHANGELOG auto-merged): 23 of 23 tests pass.

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