Fix dpnp.tensor.expm1 handling of complex(+-0, 0)

[vlad-perevezentsev]

This PR proposes to fix issue #2878 where dpnp.tensor.expm1(complex(-0.0, 0.0)) returned -0.0 + 0.0j on CPU instead of 0.0 + 0.0j as required by the Array API specification

The fix adds an explicit special-case branch for complex(±0, 0) in expm1.hpp so CPU and GPU behavior are now consistent and compliant with the specification.

Also test_expm1_special_cases test was extended to cover complex(-0.0, 0.0)

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2926/index.html

[antonwolfy]

do we need to validate the sign in other cases also when imag=0.0 ?

[coveralls]

coverage: 78.248%. remained the same — fix_expm1_complex into master

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev0=py313h509198e_43 ran successfully.
Passed: 1358
Failed: 2
Skipped: 16