fix: repair the tests workflow (FileWidget str paths, Windows build, stale test API)

[kipp-ing]

The tests workflow fails on development for several independent reasons. This fixes the ones with a clear cause. The mypy job still fails on Python 3.10 and 3.13, so the tests check as a whole stays red there (see below).

• FileWidget with a str path. FileWidget.__init__ read file_name.suffix before converting file_name to a Path, so every GUI test that passes a str (the test helper does, via shutil.copy) failed with AttributeError: 'str' object has no attribute 'suffix'. The regression came in with fix: handle None channel comment when encoding metadata #1301. The Path conversion now happens first.
• Stale test API. test_FileWidget_TabModifyAndExport used the QListWidget API (.count(), .item()) on selected_filter_channels, which is a QTreeWidget in the file widget. It was hidden behind the suffix error. It now uses topLevelItemCount() / topLevelItem() / text(0).
• Windows test jobs. They failed building the sdist with CMAKE_C_COMPILER not set (CMake picks the NMake generator). Same setup steps as pyinstaller_build from fix: unblock test collection and Windows pyinstaller build #1310 (Ninja + Visual Studio 2022 Build Tools). With this the Windows jobs get through the build and run the test suite. Some of these steps may turn out to be redundant and can be trimmed afterwards.
• ruff format --check failed on gui/utils.py (trailing whitespace only).

Result on this PR

• pytest passes on every job that gets to run it: 223 passed on Linux, 225 passed on Windows 3.10 / 3.12 / 3.13. ruff and doc are OK.
• Windows 3.11 was killed by the 600 s pytest timeout in test_PlotWidget_ContextMenu::test_Action_CopyChannelStructure_Group, blocked in drag.exec() (gui/widgets/tree.py). The same test passes on the other three Windows jobs in the same run, so this looks like a flaky drag-and-drop hang unrelated to this change.

Not addressed

• The mypy job: ~430 errors on 3.10, exit code 2 on 3.13 (checked on Linux and Windows). It is the only thing still failing on those jobs.