Enable / document Sonar coverage for tests

[blinkerbit]

Source: gitlab-issues-export.md (Issue 7)

Weight: 2

Problem / context

sonar-project.properties excludes tests from main analysis and has commented sonar.tests / coverage paths.

Suggested approach / acceptance criteria

• Document how to generate coverage.xml and uncomment/adjust Sonar properties for teams that want coverage in SonarQube.
• Optional: CI job that uploads coverage (if CI issue is done).