fix: let open_like create arrays by default

[goutamadwant]

Summary

Closes #3352.

zarr.api.asynchronous.open_like is intended to create or open a persistent array using metadata copied from an existing array. When callers omitted mode, or passed mode=None, it delegated to open_array without a create-capable mode and raised ArrayNotFoundError for a missing target path.

This changes open_like so an omitted or None mode defaults to "a" before delegating to open_array. Explicit modes are still passed through unchanged.

The regression test covers both Zarr v2 and v3, with mode omitted and with mode=None.

For reviewers

The main behavior choice is defaulting only open_like to append/create mode. I left open_array unchanged so this stays focused on the helper whose purpose is to create a similar persistent array.

Verified with:

• uv run pytest -q tests/test_api.py::test_open_like_creates_array_by_default tests/test_api.py::test_array_like_creation --tb=short
• uv run pytest -q tests/test_api.py --tb=short
• uvx prek run --files src/zarr/api/asynchronous.py tests/test_api.py changes/3352.bugfix.md

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.

AI coding assistance is welcome, but a human must be the author and is responsible for the contents of the PR. The description and any review responses must be in your own words. Please read AI-assisted contributions before opening.

TODO

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions - N/A, no new public API
• New/modified features documented in docs/user-guide/*.md - N/A, bug fix only
• 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 (d916f75) to head (2af64ca).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4146   +/-   ##
=======================================
  Coverage   93.84%   93.84%           
=======================================
  Files          91       91           
  Lines       12570    12572    +2     
=======================================
+ Hits        11796    11798    +2     
  Misses        774      774           

Files with missing lines	Coverage Δ
src/zarr/api/asynchronous.py	96.32% <100.00%> (+0.02%)	⬆️

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[d-v-b]

looks good, thanks for fixing this.

requested changes

• can you add a test that probes the intersection between a read-only store and a mutable access mode (like "a")? We expect this to error.
• can you add a note to the changelog that users with a read-only store will need to explicitly declare mode="r" when using these routines
• can you add docstrings to the open_like routines that explain what the default mode is.