Align dpnp.tensor.expand_dims with Python array API 2025.12

[antonwolfy]

This PR aligns dpnp.tensor.expand_dims with the Python array API specification (revision 2025.12), which defines the signature as expand_dims(x, /, axis) — axis is a required positional-or-keyword argument with no default.

Previously the tensor-level function used expand_dims(X, /, *, axis=0) (axis keyword-only with a default of 0), which did not match the standard.
The public dpnp.expand_dims wrapper is also made positional-only for its array argument ((a, /, axis)), consistent with sibling array-API functions in the same module (squeeze, reshape, broadcast_to, stack).

The function body was already multi-axis capable; this change updates the signature and documentation to match, and closes gaps in test coverage.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2988/index.html

[coveralls]

coverage: 78.11%. remained the same — align-tensor.expand_dims-with-array-api-2025.12 into master

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev2=py313h509198e_17 ran successfully.
Passed: 1372
Failed: 3
Skipped: 5