Improvement: Check that PRs to be merged have no merge commits

### What

When creating merge commits, conflicts might arise. If that is the case, and conflicts need to be resolved, it might be hard to see in a pull request what was change. Because of that, to reduce the chance that unintentional changes are merged, we should forbid merge commits in pull requests.

### How

_No response_

### Estimates for realization

_No response_

### Category

- [ ] Affects Detailed Design

### Requirements / Architecture

- [x] Requirements / Architecture are not affected by this change?

Start by inspecting the repository’s GitHub configuration and workflow files for pull-request merge rules. Reproduce a pull request containing a merge commit and identify the existing validation or protection entry point. Done means such pull requests are reliably rejected or blocked, with the behavior verified through the relevant GitHub check or configuration.

Written by the indexing model from the issue text.