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!!