Fix CI: install pytest before ty check so imports resolve

The lint job runs `ty check src tests` but only had runtime deps
installed. pytest was missing, causing unresolved-import errors.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: d-krupke

.github/workflows/pytest.yml

@@ -22,6 +22,7 @@ jobs:
           ruff check src tests
       - name: Type check with ty
         run: |
+          pip install pytest
           pip install -e .
           ty check src tests