Studio: run the src-tauri unit tests in CI and fix the two that never ran (#7558)

studio-tauri-smoke.yml only ever built the crate, so none of its ~100 unit
tests executed. Running them surfaced two that were broken on platforms CI
never exercised:

- non_utf8_import_name_preserves_csv_extension built a filename containing a
  raw 0xFF byte. Linux stores that fine, macOS enforces UTF-8 on APFS/HFS+ and
  refuses to create it, so the test panicked on the unwrap. Skip when the
  filesystem rejects the name; the branch under test is only reachable where
  such a file can exist.

- losing_a_studio_package_changes_the_fingerprint created the posix venv
  layout unconditionally, but site_packages_dirs() only walks
  lib/<pyver>/site-packages on unix and looks at Lib/site-packages on Windows.
  The dist-info was therefore invisible to the fingerprint there, removing it
  changed nothing and the assert_ne could never hold. Build the layout the
  code actually reads for the target platform.

Add the cargo test step to the existing Tauri job, where the toolchain and
WebKit dev packages are already installed.

.github/workflows/studio-tauri-smoke.yml

@@ -91,6 +91,16 @@ jobs:
           npm run build
           test -f dist/index.html
 
+      # The crate carries ~100 unit tests (native_file_dialogs, preflight,
+      # install, desktop_auth, ...) that nothing ran until now: this workflow
+      # only ever built. Run them here, where the toolchain and the WebKit dev
+      # packages are already installed, so a broken assertion fails the PR
+      # instead of sitting unnoticed. `--no-fail-fast` reports every failing
+      # test in one run rather than stopping at the first.
+      - name: Rust unit tests (studio/src-tauri)
+        working-directory: studio/src-tauri
+        run: cargo test --no-fail-fast
+
       - name: Tauri debug build (Linux, no bundle, no codesign)
         # `--debug` + `--no-bundle` keeps this lean: compiles the Rust crate,
         # confirms the frontend dist is wired into Tauri, but skips the AppImage