fix: reject pickling writable ZipStore

[AMBRA7592]

Summary

Pickling a ZipStore doesn't serialize the archive; it serializes the parameters to reopen it. So pickling is safe exactly when reopening is safe.

For readers (mode="r"), it is: many independent processes can open the same path and just read. Round-trips fine.

For writers, it isn't. A ZIP has one central directory written at close. Fan out several unpickled writers to one path and each rewrites that directory on close; last close wins, the rest is corruption. Since cross-process fan-out is why you pickle at all, the writer pickle path leads straight into the one thing ZIP can't survive. Rejecting at __getstate__ fails fast instead.

Closes #3516.

For reviewers

Keying on _zmode != "r" (not read_only) is right: an append-mode store can be read_only=True yet still rewrite its directory on reopen. Only "r" reopens harmlessly.

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

TODO

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.84%. Comparing base (ecc2d77) to head (64e98cc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4168   +/-   ##
=======================================
  Coverage   93.84%   93.84%           
=======================================
  Files          91       91           
  Lines       12549    12551    +2     
=======================================
+ Hits        11777    11779    +2     
  Misses        772      772           

Files with missing lines	Coverage Δ
src/zarr/storage/_zip.py	97.74% <100.00%> (+0.02%)	⬆️
src/zarr/testing/store.py	98.91% <100.00%> (ø)

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