Fix meshgrid return type

[antonwolfy]

This PR fixes dpnp.meshgrid to always return tuple instead of list.

Changes dpnp.meshgrid() to consistently return a tuple to match NumPy's behavior (version 2.5+) and align with the newer version 2025.12 of Python array API spec.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2854/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev0=py313h509198e_6 ran successfully.
Passed: 1355
Failed: 5
Skipped: 16

[coveralls]

coverage: 78.429% (-0.003%) from 78.432% — fix-meshgrid-return-type into master