TST: add test for dtype argument in str.decode

[hippowm]

This PR adds a test case for str.decode and ensures it correctly infers the string datatype, when dtype=None and the option future.infer_string is used. This argument was introduced in PR #60940 but has no test for None. This test adds coverage to the following line:

    def decode(
        self, encoding, errors: str = "strict", dtype: str | DtypeObj | None = None
    ):
        ...
        if dtype is not None and not is_string_dtype(dtype):
            raise ValueError(f"dtype must be string or object, got {dtype=}")
        if dtype is None and get_option("future.infer_string"):
            dtype = "str" #✅ NOW COVERED
        # TODO: Add a similar _bytes interface.
        if encoding in _cpython_optimized_decoders:
            # CPython optimized implementation
            f = lambda x: x.decode(encoding, errors)
        ...

Note: Parts of this test have been automatically generated by a novel technique that we're currently developing as part of an academic research project aiming at improving test coverage. To not waste developer time, two researchers manually checked the test before submitting it. We appreciate the developers' time and any feedback is welcomed.

[mroeschke]

Suggested change

	ser = Series([b"a", b"b", b"c"]) # Use byte strings
	ser = Series([b"a", b"b", b"c"])

[mroeschke]

Please use the option_context context manager

[mroeschke]

Suggested change

	ser = Series([b"a", b"b", b"c"]) # Use byte strings
	ser = Series([b"a", b"b", b"c"])