ci: run java when test files change, correct merge_group usage

[ewanharris]

Description

Corrects the Build and Test (Java) workflow to use the glob syntax to ensure any file under tests causes tests to run. Also corrects the merge_group usage to match the docs, it doesn't support paths, only an activity type.

References

Review Checklist

• [x] I have clicked on "allow edits by maintainers".
• [ ] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• [x] The correct base branch is being used, if not main
• [ ] I have added tests to validate that the change in functionality is working as expected