Remove special mapping of auto to {} in open_zarr

[jsignell]

• Closes Inconsistent chunking between xr.open_zarr and xr.open_dataset(..., engine='zarr') with chunks="auto" #11002
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

This PR makes the handling of chunks="auto" consistent between open_zarr and open_dataset(..., engine="zarr").

The handling of chunks still differs in open_zarr vs open_dataset(..., engine="zarr") in that the default in open_zarr is to use chunks="auto" and a chunk manager (aka dask) when available in your env. And in open_dataset the default is to use chunks=None (aka no chunks).

[dcherian]

Someone is going to complain but I think this is an improvement.

• it removes one source of confusion.
• in general you do want a multiple of the on-disk chunks.
• users can still pass {} to get previous behaviour.

^ perhaps we can call that third point out in the release note.

Also, since this could be pretty breaking I'd like us to get a few more thumbs up on it.

[jsignell]

since this could be pretty breaking I'd like us to get a few more thumbs up on it.

Yeah absolutely.

I am wondering if it's worth trying to improve the auto logic to make it less willing to split chunks before we merge this. If auto never split chunks then I think this is a much less breaking change. As it currently stands, auto prefers to keep existing chunk boundaries, but it isn't strict about it. I would kind of like something that is logically (max({}, "auto"), but maybe that should be a different name?