Handle negative imaginary parts in number_to_string (#551)

[koriyoshi2041]

Fixes #551.

number_to_string reconstructs complex numbers by formatting then re-parsing:

number = number.__class__(
    "{real}+{imag}j".format(real=..., imag=...)
)

When the imaginary part is negative, the formatted imag already starts with -, so the literal + produces an invalid string such as "1.0+-1.0j", and complex("1.0+-1.0j") raises ValueError: complex() arg is a malformed string.

>>> from deepdiff.helper import number_to_string
>>> number_to_string(1-1j, significant_digits=1)
ValueError: complex() arg is a malformed string
>>> from deepdiff import DeepHash
>>> DeepHash(1-1j, significant_digits=3)   # same crash

The fix joins real and imag using the imaginary part's own sign, so negative imaginary parts produce "1.0-1.0j" instead of "1.0+-1.0j". Positive parts are unchanged.

The existing test_number_to_string_complex_digits cases only exercised non-negative (or rounds-to-zero) imaginary parts. Added two cases with non-zero negative imaginary parts (which raised before the fix); the full parametrized test passes.