Fixes the failing test `test_is_split_into_words` in `test_pipelines_token_classification.py`
Author: st81
Pull request: yes

# What does this PR do?

This test was originally added in https://github.com/huggingface/transformers/pull/38818 .  
However, running the test currently results in a failure:

```bash
RUN_SLOW=1 python -m pytest -k "test_is_split_into_words" tests/pipelines/test_pipelines_token_classification.py
````

First assertion failure:

```bash
E AssertionError: Lists differ: [[{'entity_group': 'PER', 'score': 0.921, [121 chars]29}]] != [{'entity_group': 'PER', 'score': ANY(floa[129 chars] 29}]
E
E First differing element 0:
E [{'entity_group': 'PER', 'score': 0.921, [120 chars] 29}]
E {'entity_group': 'PER', 'score': ANY(floa[38 chars]: 11}
E
E Second list contains 1 additional elements.
E First extra element 1:
E {'entity_group': 'LOC', 'score': ANY(float), 'word': 'New York', 'start': 21, 'end': 29}
E
E - [[{'end': 11,
E ? -
E
E + [{'end': 11,
E - 'entity_group': 'PER',
E ? -
E
E + 'entity_group': 'PER',
E - 'score': 0.921,
E + 'score': ANY(float),
E - 'start': 6,
E ? -
E
E + 'start': 6,
E - 'word': 'Sarah'},
E ? -
E
E + 'word': 'Sarah'},
E - {'end': 29,
E ? -
E
E + {'end': 29,
E - 'entity_group': 'LOC',
E ? -
E
E + 'entity_group': 'LOC',
E - 'score': 0.999,
E + 'score': ANY(float),
E - 'start': 21,
E ? -
E
E + 'start': 21,
E - 'word': 'New York'}]]
E ? - -
E
E + 'word': 'New York'}]

tests/pipelines/test_pipelines_token_classification.py:331: AssertionError
```

Second assertion failure:

```bash
E       AssertionError: Lists differ: [[{'e[25 chars]re': 0.921, 'word': 'Sarah', 'start': 6, 'end'[255 chars]40}]] != [[{'e[25 chars]re': ANY(float), 'word': 'Sarah', 'start': 6, [277 chars]42}]]
E       
E       First differing element 1:
E       [{'en[24 chars]re': 0.999, 'word': 'Wolfgang', 'start': 11, '[86 chars] 40}]
E       [{'en[24 chars]re': ANY(float), 'word': 'Wolfgang', 'start': [98 chars] 42}]
E       
E       Diff is 738 characters long. Set self.maxDiff to None to see it.

tests/pipelines/test_pipelines_token_classification.py:344: AssertionError
```

After applying the fix, the same command passes successfully:

```bash
======================================================================================= test session starts ========================================================================================
platform linux -- Python 3.10.11, pytest-8.4.1, pluggy-1.6.0
rootdir: /home/shutotakahashi/projects/transformers
configfile: pyproject.toml
plugins: asyncio-1.0.0, anyio-4.9.0, xdist-3.7.0, timeout-2.4.0, rerunfailures-15.1, order-1.3.0, rich-0.2.0, hypothesis-6.135.14
asyncio: mode=strict, asyncio_default_fixture_loop_scope=function, asyncio_default_test_loop_scope=function
collected 21 items / 20 deselected / 1 selected                                                                                                                                                    

tests/pipelines/test_pipelines_token_classification.py::TokenClassificationPipelineTests::test_is_split_into_words PASSED                      
```

No changes were made to the core implementation — only the test code was modified to better reflect the intended behavior.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#create-a-pull-request),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@Rocketknight1 


Comments:
cc @yushi2006, confirmed that the test was failing before but passes with this PR, so I'm going to merge! My fault for missing it - I didn't spot that it was `@slow` so it wasn't running in the CI.

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