MNT: wrap initial_liquid_mass and initial_gas_mass with _to_float

Consistent with all other tank-kind fields — these attributes can
be Function objects in RocketPy, so they need the same scalar
extraction to avoid Pydantic validation failures.

Made-with: Cursor

Author: GabrielBarberini

src/services/flight.py

@@ -283,8 +283,8 @@ def _extract_tanks(motor) -> list[MotorTank]:
                         "liquid_mass_flow_rate_out": _f(
                             tank.liquid_mass_flow_rate_out
                         ),
-                        "initial_liquid_mass": (tank.initial_liquid_mass),
-                        "initial_gas_mass": (tank.initial_gas_mass),
+                        "initial_liquid_mass": _f(tank.initial_liquid_mass),
+                        "initial_gas_mass": _f(tank.initial_gas_mass),
                     }
                 case TankKinds.ULLAGE:
                     data["ullage"] = _f(tank.ullage)