fix(storage): add _is_open guard in ZipStore.exists to prevent AttributeError

[NIK-TIGER-BILL]

Description

ZipStore.exists() accessed self._lock directly without first checking self._is_open, causing an AttributeError when the store was instantiated but not yet opened.

All other mutating/reading methods in ZipStore (e.g. _get, _set, set, set_if_not_exists, delete) guard access to self._lock with:

if not self._is_open:
    self._sync_open()

exists() was the only method that missed this guard.

Reproduction (before fix)

import zipfile, asyncio
from zarr.storage import ZipStore

with zipfile.ZipFile("/tmp/test.zip", "w") as zf:
    zf.writestr("zarr.json", b"{}")

store = ZipStore("/tmp/test.zip", mode="r")
asyncio.run(store.exists("zarr.json"))
# AttributeError: 'ZipStore' object has no attribute '_lock'

Fix

Added the missing _is_open check at the top of exists(), consistent with all other methods.

Fixes #3588

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)