Validate str type before tuple cast

[ikyasam18]

Description

Add string type validation for tuple parameter parsing to prevent unintended string splitting behavior.

Motivation and Context

Currently, when parsing a string input like '("abcd")' with TupleParameter(), it incorrectly splits the string into individual characters:

# Current behavior (incorrect)
>>> luigi.TupleParameter().parse('("abcd")')
('a', 'b', 'c', 'd')

# Expected behavior
>>> luigi.TupleParameter().parse('("abcd",)')
('abcd',)

This change adds validation to ensure proper tuple formatting and prevents incorrect string splitting. Now it raises a ValueError when detecting improperly formatted input:

>>> luigi.TupleParameter().parse('("abcd")')
ValueError: Parsed result cannot be a string

Have you tested this? If so, how?

I have tested this change by:

1. Adding unit tests to verify the error handling for improperly formatted string inputs
2. Confirming that properly formatted tuple strings like '("abcd",)' still parse correctly
3. Verifying that a ValueError is raised for invalid inputs using the Python interpreter

>>> luigi.TupleParameter().parse('("abcd")')
Traceback (most recent call last):
  File "C:\Users\MasayukiKamoda\Desktop\DESK_work\develop\luigi\luigi\parameter.py", line 1385, in parse
    return tuple(self._convert_iterable_to_tuple(x) for x in json.loads(x, object_pairs_hook=FrozenOrderedDict))
                                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "E:\dev\anaconda3\Lib\json\__init__.py", line 359, in loads
    return cls(**kw).decode(s)
           ^^^^^^^^^^^^^^^^^^^
  File "E:\dev\anaconda3\Lib\json\decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "E:\dev\anaconda3\Lib\json\decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\MasayukiKamoda\Desktop\DESK_work\develop\luigi\luigi\parameter.py", line 1391, in parse
    raise ValueError("Parsed result cannot be a string")
ValueError: Parsed result cannot be a string

[ikyasam18]

@dlstadther
Thank you for merging the PR. I hope it will be released soon.

[dlstadther]

@ikyasam18 , this was released as apart of 3.6.0

• https://github.com/spotify/luigi/releases/tag/v3.6.0

The release is also on PyPi.