Fix diagonal() strides calculation for empty results

[antonwolfy]

Previously dpnp.diagonal() returned incorrect strides when the diagonal was empty (e.g., when offset >= array width). The stride was set to itemsize instead of the correct st_n + st_m.

This PR:

• fixes stride calculation to consistently use (st_n + st_m)
• simplifies implementation using unified formula to calculate resulting shape, strides, and offset
• extends tests coverage including scenarios with empty diagonals, views, and non-contiguous arrays

The PR closes #2761.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2814/index.html

[github-actions]

Array API standard conformance tests for dpnp=0.20.0dev4=py313h509198e_6 ran successfully.
Passed: 1355
Failed: 2
Skipped: 16

[coveralls]

coverage: 81.165% (-0.005%) from 81.17%
when pulling b3d712d on resolve-gh-2761
into 4b64466 on master.