fix(ci): install package before running prek in prek-hook job

The prek-hook CI job was failing because mdformat_mkdocs wasn't
installed before running prek with the test configuration. The
.pre-commit-test.yaml uses local hooks that require the package
to be installed.

This fix adds the package installation step (using flit) before
the prek installation, matching the pattern used in the tests job.

Fixes: https://github.com/KyleKing/mdformat-mkdocs/actions/runs/19562228686

Author: claude

.github/workflows/tests.yml

@@ -56,6 +56,10 @@ jobs:
         with:
           python-version: 3.12
       - name: Installation (deps and package)
+        run: |
+          pip install flit~=3.10.1
+          flit install --deps=production --pth-file
+      - name: Install prek
         uses: j178/prek-action@v1
         with:
           install-only: true