Clears the ty diff bot's warnings on the new test: pass real callables to
build_dashboard_parser (not object()) and replace the pytest.mark.parametrize
with a plain loop so the file is stdlib-only.
Add a focused contract test for the headless `serve` command (routes to the
shared dashboard handler, headless by default while `dashboard` is not, accepts
the legacy --no-open, shares the same runtime/lifecycle flag surface). Also
refresh the dashboard.py module docstring to cover both commands.