fix: change None only field types to Literal[None]

This is to fix the error from Pydantic v2.12+ like the one below:

```python
.nox/test/lib/python3.14/site-packages/supabase/__init__.py:1: in <module>
    from supabase_auth.errors import (
.nox/test/lib/python3.14/site-packages/supabase_auth/__init__.py:3: in <module>
    from ._async.gotrue_admin_api import AsyncGoTrueAdminAPI  # type: ignore # noqa: F401
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.nox/test/lib/python3.14/site-packages/supabase_auth/_async/gotrue_admin_api.py:6: in <module>
    from ..helpers import (
.nox/test/lib/python3.14/site-packages/supabase_auth/helpers.py:27: in <module>
    from .types import (
.nox/test/lib/python3.14/site-packages/supabase_auth/types.py:104: in <module>
    class AuthOtpResponse(BaseModel):
.nox/test/lib/python3.14/site-packages/pydantic/_internal/_model_construction.py:242: in __new__
    set_model_fields(cls, config_wrapper=config_wrapper, ns_resolver=ns_resolver)
.nox/test/lib/python3.14/site-packages/pydantic/_internal/_model_construction.py:566: in set_model_fields
    fields, class_vars = collect_model_fields(cls, config_wrapper, ns_resolver, typevars_map=typevars_map)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.nox/test/lib/python3.14/site-packages/pydantic/_internal/_fields.py:364: in collect_model_fields
    field_info = FieldInfo_.from_annotated_attribute(ann_type, assigned_value, _source=AnnotationSource.CLASS)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.nox/test/lib/python3.14/site-packages/pydantic/fields.py:382: in from_annotated_attribute
    raise PydanticUserError(
E   pydantic.errors.PydanticUserError: Error when building FieldInfo from annotated attribute. Make sure you don't have any field name clashing with a type annotation.
E
E   For further information visit https://errors.pydantic.dev/2.12/u/unevaluable-type-annotation
```

Author: vite-falcon

src/auth/src/supabase_auth/types.py

@@ -102,8 +102,8 @@ class AuthResponse(BaseModel):
 
 
 class AuthOtpResponse(BaseModel):
-    user: None = None
-    session: None = None
+    user: Literal[None] = None
+    session: Literal[None] = None
     message_id: Optional[str] = None