Fix .chunks loading lazy backed array data

[dcherian]

• Closes Accessing chunks on zarr backed xarray seems to load entire array into memory #6538
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

@shoyer is there a way to test this?

[shoyer]

We have some tests with InaccessibleVariableDataStore for this sort of thing, but I don't know immediately how to hook that into the Zarr backend.

[shoyer]

The simplest option would probably be a custom Zarr store that raises an error if you try to look at array data. This could be implemented as a subclass of an existing Zarr store (e.g., the in memory store) that raises an error in __getitem__ is the filename of requests does not start with ..

[dcherian]

We have some tests with InaccessibleVariableDataStore for this sort of thing,

great tip! We don't need zarr actually, this was triggering loading of all lazy arrays.

[andersy005]

Thank you, @dcherian!

[rabernat]

Awesome work!