fix: invalid dataclass mutable default

[ds-filipknefel]

Importing TextRegion from unstructured_inference.inference.elements causes the following error

  File "<stdin>", line 1, in <module>
  File "/home/filip/Unstructured/unstructured-inference/unstructured_inference/inference/elements.py", line 209, in <module>
    @dataclass
     ^^^^^^^^^
  File "/home/filip/.pyenv/versions/3.12.1/lib/python3.12/dataclasses.py", line 1266, in dataclass
    return wrap(cls)
           ^^^^^^^^^
  File "/home/filip/.pyenv/versions/3.12.1/lib/python3.12/dataclasses.py", line 1256, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/filip/.pyenv/versions/3.12.1/lib/python3.12/dataclasses.py", line 994, in _process_class
    cls_fields.append(_get_field(cls, name, type, kw_only))
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/filip/.pyenv/versions/3.12.1/lib/python3.12/dataclasses.py", line 852, in _get_field
    raise ValueError(f'mutable default {type(f.default)} for field '
ValueError: mutable default <class 'numpy.ndarray'> for field texts is not allowed: use default_factory

this PR modifies the default value to be assigned via default_factory as instructed.