test: parallelize test suite with pytest-xdist

~2min sequential runs were painful. Added pytest-xdist and -n auto
to run across all available cores. Tests already isolate state via
tmp_path fixtures so no changes needed to test code.

Local: 2677 passed in ~30s. CI gets 4 vCPUs on ubuntu-latest.

.github/workflows/tests.yml

@@ -34,7 +34,7 @@ jobs:
       - name: Run tests
         run: |
           source .venv/bin/activate
-          python -m pytest tests/ -q --ignore=tests/integration --tb=short
+          python -m pytest tests/ -q --ignore=tests/integration --tb=short -n auto
         env:
           # Ensure tests don't accidentally call real APIs
           OPENROUTER_API_KEY: ""