Remove --system flag and use uv run --with properly

Use uv's isolated environment approach instead of system-wide
installation. This is the recommended pattern for both GitHub
Actions and GitLab CI.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: RonnyPfannschmidt

.github/workflows/test.yml

@@ -45,18 +45,15 @@ jobs:
       - name: Set up Python ${{ matrix.python }}
         run: uv python install ${{ matrix.python }}
 
-      - name: Install package from wheel
-        run: uv pip install --system dist/*.whl
-
       - name: Download test files
         uses: actions/checkout@v4
         with:
           sparse-checkout: |
             testing
           sparse-checkout-cone-mode: false
 
-      - name: Run tests
-        run: uv run pytest testing --color=yes
+      - name: Run tests with built wheel
+        run: uv run --with dist/*.whl --with pytest pytest testing --color=yes
 
   publish-to-pypi:
     name: Publish to PyPI