Raise the intended error when a space type has no deserializer

[DenisDrobyshev]

Description

deserialize_space reports an unregistered space type with the wrong error:

>>> from minari.serialization import deserialize_space
>>> deserialize_space({"type": "Graph"})
TypeError: deserialize_space() missing 1 required positional argument: 'space_dict'

type_value_dispatch builds its table as defaultdict(func). defaultdict
calls its factory with no arguments, so func — the fallback, which takes
space_dict — was invoked without it. The NotImplementedError it is written
to raise never got the chance:

@type_value_dispatch
def deserialize_space(space_dict: Dict) -> spaces.Space:
    raise NotImplementedError(
        f"No deserialization method available for {space_dict['type']}"
    )

The factory now returns the fallback rather than being it, and the message
names the type that has no deserializer.

The two directions disagreed until now: serialize_space is a singledispatch
and reports unsupported spaces correctly, which is what
test_space_serialize_deserialize_unsupported covers. Nothing exercised the
deserializing direction, so this went unnoticed.

Tests

tests/test_serialization.py gains test_space_deserialize_unsupported,
parametrised over Graph, Sequence and OneOf — three Gymnasium spaces with
no deserializer registered. All three fail on main with the TypeError above
and pass here; the file goes from 19 to 22 passing.

pre-commit run is clean on both files. tests/test_namespace.py fails 9 cases
here both with and without this change — they reach the remote storage, and the
baseline is the same.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have run the pre-commit checks with pre-commit run --all-files
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective