Refactor document name sanitization for consistent filename generation

- Introduced FdsDocumentNames class to centralize the logic for sanitizing document names used in downloads.
- Updated FdsMfr to utilize the new SafeFileName method for generating safe filenames.
- Enhanced documentation for FdsDocumentNames and its methods to clarify usage and purpose.
- Added global.json file to configure testing framework settings.

.github/copilot-instructions.md

@@ -14,7 +14,7 @@
 - Routes: `/{fn?}/{id?}/{code?}` → `IntranetController.Index`; `/do/{fn?}/{id?}/{code?}` → `IntranetController.Do`.
 - Build app: `dotnet build Fuchs/Fuchs.csproj -c Debug`. Build all: `dotnet build Fuchs_Intranet.slnx -c Debug`.
 - Frontend assets are source-built: run the gulp tasks in `Fuchs/` (`npx gulp min`, or `npx gulp all` when copied/static assets also need refreshing) whenever JS or SCSS/CSS sources change. The generated files under `Fuchs/wwwroot/web/` are what the app serves.
-- Test: `dotnet test Fuchs.Tests/Fuchs.Tests.csproj -c Debug`.
+- Test: `dotnet test --project Fuchs.Tests/Fuchs.Tests.csproj -c Debug` (the root `global.json` opts `dotnet test` into the Microsoft.Testing.Platform runner that xunit.v3 needs on the .NET 10 SDK; the positional-path form no longer works).
 - Always build **and** run the test suite before committing. CA1416 (platform-compatibility) is suppressed solution-wide via the root `.editorconfig` — the app is Windows-only. The build may still emit other pre-existing analyzer warnings; only treat `: error` lines as failures.
 - Submodules include the OCORE projects and `eRechnungLib` (ZUGFeRD/Factur-X + XRechnung generation) — invoices are moving to eRechnung output.
 - Project structure (relative to `Fuchs/`):