fix: suggest store class when reading a packed zarr store by path

[JOhnsonKC201]

• Closes anndata.io.read_zarr incompatible with a Zarr ZipStorage #2152
• Tests added
• Release note not necessary because:

Problem

zarr.open treats a path like adata.zarr.zip as a directory store, finds no
group in it, and raises a bare GroupNotFoundError. Nothing in that error hints
that the path has to be wrapped in a store class first, so the fix is hard to
guess:

adata.write_zarr(zarr.storage.ZipStore("adata.zarr.zip", mode="w"))

ad.io.read_zarr("adata.zarr.zip")
# zarr.errors.GroupNotFoundError: No group found in store
# StorePath(LocalStore, 'file://…/adata.zarr.zip') at path ''

Approach

This follows @flying-sheep's suggestion in
#2152 (comment). When
read_zarr gets a path-like whose suffix is known to hold a whole store in a
single file, the raised error gets a note naming the store class to use:

Did you mean `read_zarr(zarr.storage.ZipStore('adata.zarr.zip'))`?

Deliberately not done, per @ilan-gold's comment on the same issue: the
suffix is not used to open a ZipStore automatically. Reading such a path still
raises, so there is no magic-string store inference and no behaviour change
beyond the added note. add_note is already the codebase's idiom for enriching
I/O errors (anndata._io.utils.add_key_note).

The mapping from suffix to store class is a module-level constant, so other
single-file stores can be added later without touching the logic.

read_lazy opens user-supplied paths the same way
(anndata/experimental/backed/_io.py) and so has the same rough edge. I left it
alone to keep this focused on the reported issue. Happy to extend it there in
this PR or a follow-up, whichever you prefer.

Test plan

• test_read_zarr_zip_path_suggests_store: asserts the note is attached,
  and that the incantation it suggests actually round-trips
• test_read_zarr_dir_path_suggests_nothing: a directory store that holds
  no group gets no misleading suggestion
• Both verified to fail before the source change and pass after
• Full suite (-m "not dask_distributed"): 8051 passed, 1441 skipped,
  210 xfailed, 154 subtests passed in 7:45. The 3 failures are pre-existing
  and unrelated (all reproduce on main, and none touch this code path):
  test_write_large_categorical[zarr2/zarr3] is an OOM from running 16
  xdist workers locally and passes when run serially;
  test_concatenate_disk.py::test_output_dir_exists passes a Windows path
  to pytest.raises(match=...), which is an invalid regex (incomplete escape \U) on Windows only.
• dask_distributed shard: 59 passed, 6 skipped
• ruff check, ruff format --check, ruff check --preview --select=PLR0917,
  and codespell clean on the changed files

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.90%. Comparing base (f723aa8) to head (16e4573).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2571      +/-   ##
==========================================
- Coverage   87.78%   85.90%   -1.88%     
==========================================
  Files          49       49              
  Lines        7799     7816      +17     
==========================================
- Hits         6846     6714     -132     
- Misses        953     1102     +149     

Files with missing lines	Coverage Δ
src/anndata/_io/zarr.py	84.61% <100.00%> (+2.61%)	⬆️

... and 9 files with indirect coverage changes