Add simple_efficiency tests for numpy array inputs

[aman-coder03]

this PR adds test coverage for vectorized use of pvlib.transformer.simple_efficiency with numpy.ndarray inputs.

the tests verify:

• support for NumPy array inputs for input_power, load_loss,
  and no_load_loss
• consistency between vectorized and scalar evaluations

No functional changes are introduced.

fixes #2649

[aman-coder03]

the CI failures are due to deprecation-enforcement tests in tests/test_irradiance.py, unrelated to this PR.
this PR only modifies tests/test_transformer.py.
please let me know if you would like me to rebase

[cwhanse]

@aman-coder03 this is the scope of #2658

Could you complete #2646 first? It needs a whatsnew note.

[aman-coder03]

@cwhanse shall I create pvlib-python\docs\sphinx\source\whatsnew\v0.13.2.rst ?
and then add

* Fix a division-by-zero condition in
  :py:func:`~pvlib.transformer.simple_efficiency` when ``load_loss = 0``.
  (:issue:`2645`)

in Bug Fixes

[cwhanse]

It should be v0.14.1.rst. Thanks!

[aman-coder03]

@cwhanse i have completed that, you can have a look!!

[aman-coder03]

@cwhanse as suggested by you in #2646 , i have implemented the additional tests in this separate PR to avoid scope creep in #2646. Let me know if you would like any adjustments.

[cwhanse]

@aman-coder03 I think it is premature to test with all inputs as np.array. In other discussions we don't have consensus to allow the parameters no_load_loss and load_loss to be anything other than float, and intend to edit the docstring accordingly.

If we agree in the future to allow vectors for these parameters, then this test would be needed.

I don't want to include it now so to prevent someone's AI from deciding we have missed a combination of types in other tests.