`zarr.Array.__len__` removed in v3 rewrite

[h-mayorquin]

zarr.Array in v2 implemented __len__, returning shape[0] for dimensioned arrays and raising TypeError for 0-dimensional arrays, matching numpy's behavior exactly:

zarr-python/zarr/core.py

Lines 649 to 654 in 3ad97b9

	def __len__(self):
	if self.shape:
	return self.shape[0]
	else:
	# 0-dimensional array, same error message as numpy
	raise TypeError("len() of unsized object")

In zarr v3, __len__ is not defined on Array, so len(array) raises TypeError: object of type 'Array' has no len() for all arrays regardless of dimensionality.

We encountered this while migrating hdmf-zarr to zarr v3 (PR #325). Our code uses hasattr(obj, "__len__") and len() calls to distinguish arrays from scalars, which previously worked consistently across numpy arrays, h5py datasets, and zarr v2 arrays.

I searched the zarr-python issue tracker for any prior discussion of __len__ on Array but could not find something very specific. This change is also not mentioned in the v3 migration guide. I suspect it was dropped to align with the Python array API standard, which does not include __len__ on array objects. My main interest in filing this is so it is documented and discoverable for anyone who runs into it while migrating from v2.