Refine dataclass check

Change the `dataclasses.is_dataclass` overload used from
```
@overload
def is_dataclass(obj: object) -> TypeIs[DataclassInstance | type[DataclassInstance]]: ...
```
to
```
@overload
def is_dataclass(obj: type) -> TypeIs[type[DataclassInstance]]: ...
```

This fixes the following mypy errors:
```
fastapi/encoders.py:244: error: Argument 1 to "asdict" has incompatible type "DataclassInstance | type[DataclassInstance]"; expected "DataclassInstance"  [arg-type]
fastapi/routing.py:122: error: Argument 1 to "asdict" has incompatible type "DataclassInstance | type[DataclassInstance]"; expected "DataclassInstance"  [arg-type]
Found 2 errors in 2 files (checked 44 source files)
```

fastapi/encoders.py

@@ -240,7 +240,7 @@ def jsonable_encoder(
             custom_encoder=encoders,
             sqlalchemy_safe=sqlalchemy_safe,
         )
-    if dataclasses.is_dataclass(obj):
+    if dataclasses.is_dataclass(type(obj)):
         obj_dict = dataclasses.asdict(obj)
         return jsonable_encoder(
             obj_dict,

fastapi/routing.py

@@ -118,7 +118,7 @@ def _prepare_response_content(
             )
             for k, v in res.items()
         }
-    elif dataclasses.is_dataclass(res):
+    elif dataclasses.is_dataclass(type(res)):
         return dataclasses.asdict(res)
     return res