[d-v-b]

This PR partially reverts the changes in #4505 and solves the underlying bug #4505 aimed to solve more directly by allowing the VerboseModule to be pickled (by defining __reduce__).

edit: I thought #4505 had been merged, but it hasn't. so this PR just closes #4029 directly without the removal component, which is slated for the 3.5.0 release

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The zarr.storage module can now be serialized with pickle and cloudpickle. Previously, code that referenced the module (for example a Dask task that used zarr.storage.ObjectStore) failed with TypeError: cannot pickle 'VerboseModule' object.

Closes #4029.

zarr.storage swaps its class for a VerboseModule subclass of ModuleType so it can warn when default_compressor is set. Serializers that dispatch on the exact type, such as cloudpickle (Dask's default), have no reduce for that subclass and fall through to object.__reduce_ex__, which raises. This PR gives VerboseModule a __reduce__ that re-imports the module by name, so both pickle and cloudpickle round-trip it to the same module object.

The deprecation warning for zarr.storage.default_compressor is unchanged, which keeps this a pure bugfix suitable for a patch release. #4505 fixes the same bug by removing VerboseModule and the warning altogether. The versioning policy in docs/contributing.md excludes behavior changes from patch releases, so that removal fits a minor release, and it can land there on top of this fix.

Testing:

• New test test_storage_module_pickles_by_name in tests/test_store/test_core.py round-trips zarr.storage through the standard library's pickle, so it needs no new test dependency. It fails on main with the TypeError above.
• cloudpickle round-trips zarr.storage with this change, checked in a throwaway environment.
• tests/test_store/test_core.py, tests/test_v2.py, and tests/test_api.py pass, including the existing default_compressor deprecation-warning test. Ruff, mypy, and the other pre-commit hooks pass.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.71%. Comparing base (957a5f6) to head (04363dc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4557   +/-   ##
=======================================
  Coverage   94.71%   94.71%           
=======================================
  Files          94       94           
  Lines       13663    13667    +4     
=======================================
+ Hits        12941    12945    +4     
  Misses        722      722           

Files with missing lines	Coverage Δ
src/zarr/storage/__init__.py	95.83% <100.00%> (+0.83%)	⬆️

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