IndexError when saving few large examples to disk

### Describe the bug

I ran into this issue when processing a file (900MB) with just one example but simplified for a quicker reproducer below. The problem is that, if `num_shards` is not explicitly set, we calculate it manually using https://github.com/huggingface/datasets/blob/main/src/datasets/utils/py_utils.py#L96 with the default `config.MAX_SHARD_SIZE` which is 500MB. If a single example is now larger than this, we run into an index error since the shards should be processed individually.

An easy workaround is:

`dataset.save_to_disk(output_path, max_shard_size="1GB")` or `dataset.save_to_disk(output_path, num_shards=1)`.

I believe this should be fixed and can happen in edge cases for image data, especially when just testing single partitions. The fix would be rather easy, just using a `num_shards = min(num_examples, <previously_calculated_num_shards>)`


### Steps to reproduce the bug

```python
from datasets import Dataset

target_size = 2 * 1024 * 1024  # 2 MB in bytes

base_text = (
    "This is a sample sentence that will be repeated many times to create a large dataset. "
    * 100
)
large_text = ""

while len(large_text.encode("utf-8")) < target_size:
    large_text += base_text

actual_size = len(large_text.encode("utf-8"))
size_mb = actual_size / (1024 * 1024)

data = {"text": [large_text], "label": [0], "id": [1]}

dataset = Dataset.from_dict(data)

output_path = "./sample_dataset"
# make sure this is split into 2 shards
dataset.save_to_disk(output_path, max_shard_size="1MB")
```
this results in
```

```bash
Saving the dataset (1/3 shards): 100%|████████████████████████████████████████████████████████████████████████████████| 1/1 [00:00<00:00, 162.96 examples/s]
Traceback (most recent call last):
  File "/home/tpitters/programming/toy-mmu/create_dataset.py", line 27, in <module>
    dataset.save_to_disk(output_path, max_shard_size="1MB")
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tpitters/programming/toy-mmu/.venv/lib/python3.13/site-packages/datasets/arrow_dataset.py", line 1640, in save_to_disk
    for kwargs in kwargs_per_job:
                  ^^^^^^^^^^^^^^
  File "/home/tpitters/programming/toy-mmu/.venv/lib/python3.13/site-packages/datasets/arrow_dataset.py", line 1617, in <genexpr>
    "shard": self.shard(num_shards=num_shards, index=shard_idx, contiguous=True),
             ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tpitters/programming/toy-mmu/.venv/lib/python3.13/site-packages/datasets/arrow_dataset.py", line 4987, in shard
    return self.select(
           ~~~~~~~~~~~^
        indices=indices,
        ^^^^^^^^^^^^^^^^
    ...<2 lines>...
        writer_batch_size=writer_batch_size,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/tpitters/programming/toy-mmu/.venv/lib/python3.13/site-packages/datasets/arrow_dataset.py", line 562, in wrapper
    out: Union["Dataset", "DatasetDict"] = func(self, *args, **kwargs)
                                           ~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tpitters/programming/toy-mmu/.venv/lib/python3.13/site-packages/datasets/fingerprint.py", line 442, in wrapper
    out = func(dataset, *args, **kwargs)
  File "/home/tpitters/programming/toy-mmu/.venv/lib/python3.13/site-packages/datasets/arrow_dataset.py", line 4104, in select
    return self._select_contiguous(start, length, new_fingerprint=new_fingerprint)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tpitters/programming/toy-mmu/.venv/lib/python3.13/site-packages/datasets/arrow_dataset.py", line 562, in wrapper
    out: Union["Dataset", "DatasetDict"] = func(self, *args, **kwargs)
                                           ~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/tpitters/programming/toy-mmu/.venv/lib/python3.13/site-packages/datasets/fingerprint.py", line 442, in wrapper
    out = func(dataset, *args, **kwargs)
  File "/home/tpitters/programming/toy-mmu/.venv/lib/python3.13/site-packages/datasets/arrow_dataset.py", line 4164, in _select_contiguous
    _check_valid_indices_value(start, len(self))
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/home/tpitters/programming/toy-mmu/.venv/lib/python3.13/site-packages/datasets/arrow_dataset.py", line 624, in _check_valid_indices_value
    raise IndexError(f"Index {index} out of range for dataset of size {size}.")
IndexError: Index 1 out of range for dataset of size 1.
```

### Expected behavior

should pass

### Environment info

datasets==4.4.2