datasets.to_list: Behaviour change in version 4.7.0 with the addition of JSON type

### Describe the bug

There is a change in the approach to decoding JSON types, added in https://github.com/huggingface/datasets/commit/d560b58e1f1ec96591a95a1780ee118f5eea6c74. Since 4.7.0, to_list() returns raw JSON strings for columns stored as Json(), while direct access via __getitem__ returns dicts. 

In versions prior to 4.7.0, to_list() returned dicts.




### Steps to reproduce the bug

1. Have datasets >= 4.7.0 installed (e.g 8.4.3)
2. Clear the datasets cache - important to reproduce.
3. Run the following:
```
from datasets import load_dataset
ds = load_dataset("google/IFEval", split="train") 
print(type(ds[0]["kwargs"][0]))  # <class 'dict'>
print(type(ds.to_list()[0]["kwargs"][0]))  # <class 'str'>
```
With datasets < 4.7.0 (e.g 4.5.0) both return dict. 

(Taken from https://github.com/UKGovernmentBEIS/inspect_evals/issues/1416)

### Expected behavior

to_list() and to_dict() doesn't perfom feature decoding, but with the addition of the Json() type, there seems to be a regression where to_list() should return dicts but now no longer does. This would break older implementations where the code assumes data can be accessed this way. 

I'm not sure what fix would be the nicest/ in alignment with the dataset package. From broadest to narrowest options:
1. decode elements of the list in the same way _get_item does - (loading all images into memory)
  would be a heavy behaviour change
2. Do a target fix for Json()
```
  def to_list(self) -> list:                                                                                                                                                                         
      pylist = query_table(
          table=self._data,                                                                                                                                                                          
          key=slice(0, len(self)),                                                                                                                                                                 
          indices=self._indices,              
      ).to_pylist()
       
      # Identify columns that are JSON type with decode set to true         
      if self._info.features:                                                                                                                                                               
          json_columns = [
              col for col, feature in self._info.features.items()                                                                                                                                    
              if isinstance(feature, Json) and feature.decode                                                                                                                                      
          ]                   
          if json_columns:                
              for row in pylist:
                  for col in json_columns:                                                                                                                                                           
                      if col in row and isinstance(row[col], str):
                          row[col] = ujson_loads(row[col])                                                                                                                                           
      return pylist 
```
3. Put a warning message in to notify of this regression, to help with debugging. 

Happy to put a PR in with the preferred option.

### Environment info

- `datasets` version: 4.8.3
- Python version: 3.13.7
- `huggingface_hub` version: 1.4.1
- PyArrow version: 23.0.1
- Pandas version: 2.2.3
- `fsspec` version: 2025.9.0


I think we can go for the same approach as in https://github.com/huggingface/datasets/pull/8122 which decodes the JSON data (and supports JSON nested in other objects)

(Or decide to decode all the features types, which I think could make sense, but this needs more careful handling so this should be discussed in another issue)



Disregard the self-assign — I missed that PR #8137 already has this covered. Sorry for the noise.