Add Codecov to workflows

[ejcsid]

Describe the solution you'd like
With CodeCov you can see CodeCoverage in PRs

Describe alternatives you've considered
It's a solution that was added in one of the it@m Projects, see digiwf-core workflow-build-services.yaml.

It's an option that can be ordered via it@m OSPO (internal documentation: https://git.muenchen.de/ccse/ospo/-/wikis/github#codecov)

Additional context
Related to an internal it@m-discussion (https://git.muenchen.de/ccse/ospo/-/issues/805)

[eidottermihi]

I'd suggest considering the use of Sonarqube Cloud, which offers a free OSS plan (see https://docs.sonarsource.com/sonarqube-cloud/administering-sonarcloud/managing-subscription/subscription-plans#oss-plan ).

Benefits from my view:

• one tool for static code analysis (incl. code coverage) instead of multiple with extra configuration (pmd, spotbugs, ... -> https://github.com/it-at-m/refarch-templates/blob/main/refarch-backend/pom.xml )
• broad support for multiple languages (https://docs.sonarsource.com/sonarqube-cloud/advanced-setup/languages/overview)
• same tool that we already use on-prem for non-OSS projects (SonarQube Server)

[ffmuc]

We are evaluating Sonarqube and CodeCov in 2026