test: migrate to pytest

Replace unittest-based coverage with pytest and update CI/docs to run the new suite.

.github/workflows/publish.yml

@@ -32,7 +32,7 @@ jobs:
         run: uv sync --frozen --no-editable
 
       - name: Run tests
-        run: uv run python -m unittest discover -s tests
+        run: uv run pytest
 
       - name: Build distributions
         run: uv build --no-sources
@@ -63,4 +63,4 @@ jobs:
       - name: Publish to PyPI
         uses: pypa/gh-action-pypi-publish@release/v1
         with:
-          packages_dir: dist
+          packages-dir: dist