fix: point two deprecation warnings at the calling code - #4612

Python shows a DeprecationWarning by default only when it is attributed
to __main__, so warnings that pointed at zarr internals were hidden
from users running scripts:

- `ArrayV2Metadata.chunk_grid` is a cached_property, which adds a frame;
  stacklevel 2 pointed at functools. Now 3.
- Setting `zarr.storage.default_compressor` used stacklevel 1, which
  pointed at zarr itself. Now 2.

Tests now assert the warning location. The deprecation policy in
docs/contributing.md explains why the location matters, and notes
that a warning raised inside a coroutine cannot reach the caller
through the synchronous API, so deprecations that must reach script
users should warn from the public synchronous function.

Co-Authored-By: Claude Opus 5.5 <[email protected]>