fix(cryo_tank): use helium-only CoolProp pressure model

tests/cryo_tank/test_tank_model.py

@@ -69,10 +69,9 @@ class TestInitialState:
         assert abs(info['T_liq'] - T_INIT) < 0.1
         assert abs(info['T_ull'] - T_INIT) < 1.0
 
-    def test_initial_pressure_components_sum_to_P_working(self):
+    def test_initial_pressure_is_provided_entirely_by_helium(self):
         tank = _make_tank()
         y0 = tank.initial_state()
         info = tank.derive(y0)
-        P_N2 = info['P_N2']
         P_He = info['P_He']
-        assert abs(P_N2 + P_He - P_WORKING) / P_WORKING < 1e-6
+        assert abs(P_He - P_WORKING) / P_WORKING < 1e-12