test: add pytest suite and gate Docker builds on it

Unit tests cover the SQL gate, broadcast codec, Hive writer layout,
and flight kinematics. CI runs pytest first; the simulator image build
runs tests in a dedicated stage before the final layer.

simulator/requirements.txt

@@ -1,3 +1,4 @@
 pyarrow>=16.0
 duckdb>=1.0
 pytz>=2024.1  # duckdb needs it to fetch TIMESTAMPTZ values into Python
+pytest>=8.0