fix: Incorrect to_schema generation for dataclass using ResultConverter

[Kumzy]

Not sure that is the best way to fix the issue.

Description

The following code was not working and returned SQLSpecError: `schema_type` should be a valid Dataclass, Pydantic model or Msgspec struct

#Import there

@dataclass
class SampleDataclass:
    """Sample dataclass for testing."""

    name: str
    value: int | None = None
    empty_field: Any = Empty
    meta: ClassVar[str] = "test"

data = ResultConverter.to_schema(data=SampleDataclass(name="test", value=42), schema_type=SampleDataclass)

I tried to replace the is_dataclass by the built in is_dataclass from dataclasses, it correctly identify whether the passed argument is a dataclass type (if it's a class) or an instance of a dataclass. Where the method in typing was working only when it was an instance of the class.

There was a fail also right after when trying to cast the dictionary into the dataclass.