Provide chunk_grid_space fallback in _iter_shard_coords

[melonora]

In the codebase _iter_shard_regions was already falling back to chunks in case there are no shards. Another method, iter_shard_coords did not have this fallback option. This PR adds a fallback uption for that method in case there is no shard grid.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

@d-v-b @joshmoore

[d-v-b]

we need separate methods for iterating over chunks and shards because they are not the same

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "zarr@git+https://github.com/zarr-developers/zarr-python.git@main",
# ]
# ///

import zarr

array = zarr.create_array({} ,shape=(10,), dtype='i4', chunks=(5,), shards=(10,), fill_value=0)

print(tuple(array._iter_shard_regions()))
# ((slice(0, 10, 1),),)
print(tuple(array._iter_chunk_regions()))
# ((slice(0, 5, 1),), (slice(5, 10, 1),))

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.14%. Comparing base (ee0e69a) to head (eded573).

Files with missing lines	Patch %	Lines
src/zarr/core/array.py	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3614      +/-   ##
==========================================
- Coverage   59.14%   59.14%   -0.01%     
==========================================
  Files          86       86              
  Lines       10172    10174       +2     
==========================================
+ Hits         6016     6017       +1     
- Misses       4156     4157       +1     

Files with missing lines	Coverage Δ
src/zarr/core/array.py	68.44% <57.14%> (-0.04%)	⬇️

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[melonora]

I see that, but do not see where this is used internally that it matters. Otherwise I would just provide the same fallback option for _iter_shard_coords and leave the original _iter_chunk intact.

[d-v-b]

we do not use these methods internally yet, but that's not a good reason for removing them.

also see #3573

[melonora]

we do not use these methods internally yet, but that's not a good reason for removing them.

also see #3573

Then I will revert them, but provide just the same fallback option for `_iter_shard_coords'

[d-v-b]

I'm confused by these changes -- is there a bug that this fixes? Because we should already fall back to the chunk grid for unsharded v2 arrays, via the _shard_grid_shape property

[melonora]

Thanks @d-v-b for your patience and pointing it out. Was mainly based on spotting it based on the other method.