Add a notification step for failed tests

[MakisH]

Adds a step to send a Matrix notification in case the system tests fail, similarly to https://github.com/precice/precice/blob/develop/.github/workflows/performance-regression.yml

I would like to further restrict this to scheduled runs only, but this is currently a reusable workflow and all other workflows call it (without further steps). Still, not too many manual tests should fail in the near future.

The needed secrets are already shared with this repository.

[Copilot]

Pull request overview

This PR adds a failure-only notification step to the reusable system test workflow so maintainers get a Matrix message when a system test run fails.

Changes:

• Add a Matrix notification step using precice/notify-action@v1 gated by if: ${{ failure() }}.
• Wire Matrix connection parameters via repository variables and a secret.

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[MakisH]

This actually means that we would need to share the secrets also with every repository that calls the system tests.

[MakisH]

I think this is getting complicated, without much of added value. We already have the labels and are integrating test suites in the CI of each repository.

I am also getting email notifications on every failure of the scheduled run, and I am the one that will have to fix it or trigger others.