fix: dtype inference in new_known_scalar

[pfackeldey]

closes https://github.com/dask-contrib/dask-awkward/issues/577

if a 0-dim array is passed to new_known_scalar the dtype was inferred to be np.dtype(type(array(...))) which results in object dtype. This PR directly uses the dtype of the input arg if it has a dtype attribute, which is true for array types.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.10%. Comparing base (8cb8994) to head (6062dc7). Report is 256 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #578      +/-   ##
==========================================
- Coverage   93.06%   92.10%   -0.97%     
==========================================
  Files          23       24       +1     
  Lines        3290     3598     +308     
==========================================
+ Hits         3062     3314     +252     
- Misses        228      284      +56     

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[pfackeldey]

Needs https://github.com/dask-contrib/dask-awkward/pull/579 to fix the awkward-main ci test

[pfackeldey]

ugh... there seem to be the expectation of dask-awkward that a 0-dim typetracer array is always promoted to a length-1 layout. This is something that does not happen with every other array type. I explicitly fixed this in awkward recently: https://github.com/scikit-hep/awkward/pull/3469#issuecomment-2802584138, but dask-awkward seems to rely on it...

(needs https://github.com/dask-contrib/dask-awkward/pull/580 to be fixed)