Replace OsDetector and OS assumptions with JUnit 5 @EnabledOnOs

[redcatbear]

Situation

The project currently uses a custom `OsDetector` utility (and an underlying `OsCheck` class) located in the `testutil` module to handle OS-specific test execution. This is typically done using methods like `OsDetector.assumeRunningOnWindows()` or `OsDetector.assumeRunningOnUnix()` inside test methods.

JUnit 5 provides a standard way to handle this via the `@EnabledOnOs` and `@DisabledOnOs` annotations. Using these standard annotations improves code readability, follows JUnit best practices, and allows for removing custom infrastructure code.

Implementation Hints

1. Refactor Tests:
   • Replace calls to `OsDetector.assumeRunningOnWindows()` with `@EnabledOnOs(OS.WINDOWS)` at the method or class level.
   • Replace calls to `OsDetector.assumeRunningOnUnix()` with `@EnabledOnOs({OS.LINUX, OS.MAC})` or similar, depending on the test requirements.
   • Update tests in:
     • `core/src/test/java/org/itsallcode/openfasttrace/core/importer/TestImporterService.java`
     • `core/src/test/java/org/itsallcode/openfasttrace/core/importer/tag/config/DescribedPathMatcherTest.java`
     • `reporter/html/src/test/java/org/itsallcode/openfasttrace/report/html/view/html/TestOriginLinkFormatter.java`
     • `product/src/test/java/org/itsallcode/openfasttrace/report/html/TestHtmlReportCssInlining.java`
2. Remove Utility Classes:
   • Once all references are removed, delete `org.itsallcode.openfasttrace.testutil.OsDetector` and `org.itsallcode.openfasttrace.testutil.OsCheck`.
   • Check for any other usages of `OsCheck` in production code.
3. Update Dependencies:
   • Ensure `junit-jupiter-api` is available (it is already used across the project).

Acceptance Criteria

1. `OsDetector` and `OsCheck` classes are removed from the codebase.
2. All tests that previously used `OsDetector` now use `@EnabledOnOs` or other JUnit 5 execution conditions.
3. The build passes on all target operating systems (Windows, Linux, macOS) in CI.
4. `mvn verify` completes successfully.