Serialization warnings for time and space axes

[FynnBe]

There is something odd when serializing some time and space (output) axes.
I suspect due to the serialization of the axis field, which is a Union warnings are generated.
Warnings such as:

PydanticSerializationUnexpectedValue: Expected `TimeOutputAxis` but got `SpaceOutputAxis` with value `SpaceOutputAxis(size=43, ...', unit=None, scale=1.0)` - serialized value may not be as expected
  PydanticSerializationUnexpectedValue: Expected `TimeOutputAxisWithHalo` but got `SpaceOutputAxis` with value `SpaceOutputAxis(size=43, ...', unit=None, scale=1.0)` - serialized value may not be as expected
  PydanticSerializationUnexpectedValue: Expected `TimeOutputAxis` but got `SpaceOutputAxis` with value `SpaceOutputAxis(size=123,...', unit=None, scale=1.0)` - serialized value may not be as expected
  PydanticSerializationUnexpectedValue: Expected `TimeOutputAxisWithHalo` but got `SpaceOutputAxis` with value `SpaceOutputAxis(size=123,...', unit=None, scale=1.0)` - serialized value may not be as expected
  PydanticSerializationUnexpectedValue: Expected `TimeOutputAxis` but got `SpaceOutputAxis` with value `SpaceOutputAxis(size=167,...', unit=None, scale=1.0)` - serialized value may not be as expected
  PydanticSerializationUnexpectedValue: Expected `TimeOutputAxisWithHalo` but got `SpaceOutputAxis` with value `SpaceOutputAxis(size=167,...', unit=None, scale=1.0)` - serialized value may not be as expected

See any test log for more examples.
Any round-trip serialization-deserialization test passes, but it would still be nice to make serialization more rigorous and avoid these warnings.