chore: run tests in parallel with pytest-xdist

Adds -n auto to pytest addopts so the test suite uses all available CPU
cores. Reduces local and CI run time from ~6 min to ~2 min.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jacalata

pyproject.toml

@@ -67,7 +67,7 @@ exclude = ['/bin/']
 
 [tool.pytest.ini_options]
 testpaths = ["test"]
-addopts = "--junitxml=./test.junit.xml"
+addopts = "--junitxml=./test.junit.xml -n auto"
 
 [tool.versioneer]
 VCS = "git"