FIX: Persist DataArray attrs in zarr append mode

[krokosik]

• Closes Updating DataArray attributes in a Zarr archive does not work as for Dataset #8116, Update dataset attributes and sync to disk store #3536
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

This PR fixes an inconsistency in zarr append behavior between Dataset and DataArray paths.

• DataArray.to_zarr(..., mode="a"/"a-") previously updated data but did not persist attrs for pre-existing arrays.
• The fix writes encoded attrs for existing arrays in append mode (while keeping r+ semantics unchanged).
• Added regression tests that:
  • verify DataArray attrs persist in append mode, and
  • verify Dataset/DataArray attr behavior is consistent.
    I also added a warning for region writes that update attrs in append mode.
    Rationale:
• region writes are often used for parallel/distributed chunk updates (thanks @max-sixty for mentioning that in Update Variable metadata when using 'a' mode in Zarr #8159)
• attrs are global metadata updates and can race across concurrent writers even if data regions are disjoint
• warning is non-breaking and nudges users toward safer patterns (serialize metadata updates, keep region writes data-only, or use r+ for strict in-place data updates)

Reproducer

import xarray as xr

# Dataset attrs update in append mode
ds_path = "tmp/ds"
xr.Dataset().assign_attrs(a=1).to_zarr(ds_path, mode="w")
xr.open_dataset(ds_path, engine="zarr").assign_attrs(b=2).to_zarr(ds_path, mode="a")

# DataArray attrs update in append mode
da_path = "tmp/da"
xr.DataArray([0], dims=["x"]).assign_attrs(a=1).to_zarr(da_path, mode="w")
xr.open_dataarray(da_path, engine="zarr").assign_attrs(b=2).to_zarr(da_path, mode="a")

assert xr.open_dataset(ds_path, engine="zarr").attrs == xr.open_dataarray(da_path, engine="zarr").attrs

Before this change, the DataArray append path did not persist updated attrs for existing arrays.

[krokosik]

Just read through issues and stumbled upon discussions in #8755 and #8343. My understanding is that it is intentional after all