WIP: searchsorted: allow x2 to be python scalar

[ev-br]

This is a proposed 2025.12 feature, cf data-apis/array-api#982

1. TBD: two separate tests (_searchsorted and _searchsorted_with_scalars) or fold into a single function.
2. needs a rebase on expand testing of searchsorted #393 done
3. needs implementing in array_api_strict : ENH: searchorted: support int | float scalar x2 array-api-strict#170
4. On CuPy: needs a -compat update: ENH: cupy: add a workaround for cp.searchorted 2nd argument array-api-compat#374
5. Dask: fails, needs an xfail : TST: dask: xfail searchorted_scalars test on dask array-api-compat#375

[ev-br]

Local testing:

$ ARRAY_API_TESTS_SKIP_DTYPES=uint16,uint32,uint64 ARRAY_API_TESTS_MODULE=array_api_compat.torch pytest array_api_tests/test_searching_functions.py::test_searchsorted_with_scalars --max-examples 50_000 

OK

$ ARRAY_API_TESTS_MODULE=array_api_compat.numpy pytest array_api_tests/test_searching_functions.py::test_searchsorted_with_scalars --max-examples 50_000 

OK

$ ARRAY_API_TESTS_MODULE=array_api_compat.dask.array pytest array_api_tests/test_searching_functions.py::test_searchsorted_with_scalars --max-examples 500 

>       meta = np.searchsorted(a._meta, v._meta)
                                        ^^^^^^^
E       AttributeError: 'int' object has no attribute '_meta'
E       
E       ========== FAILING CODE SNIPPET:
E       xp.searchsorted(dask.array<sort, shape=(1,), dtype=uint8, chunksize=(1,), chunktype=numpy.ndarray>, 0, sorter=None, **kw) with kw = {}
E       ====================