fix: replace runtime assert statements with explicit checks - #4363

AI-authored PR that removes runtime assert statements. closes #4362

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Summary

Removes every assert statement from runtime code under src/zarr/ and enables ruff's S101 rule so new ones are flagged. Asserts are stripped under python -O, so any check that matters for correctness or for narrowing a type must be an explicit if/raise.

Two of the 47 asserts were load-bearing:

• GroupMetadata.from_dict asserted that node_type was "group", and zarr.api.asynchronous.open caught AssertionError to fall back from array to group. Under -O that fallback silently disappeared. It now raises NodeTypeValidationError, which that call site already catches, and the AssertionError catch is gone.
• make_store asserted on mode, but the real validation lives downstream in StorePath.open. A bad user-supplied mode now raises the same ValueError there.

The rest fall into three buckets:

• Deleted (25): redundant with type annotations (isinstance(key, str) in the stores, isinstance(chunk_array, NDBuffer) in codecs) or with an existing check (Buffer.combine dtype checks the constructor already enforces).
• Restructured so mypy narrows on its own (11): locals in _get_loop and _validate_scalar_map, a hoisted sync_transform in the codec-pipeline write path, and get_array_metadata returning from each branch via two small helpers.
• Converted to explicit raises (11): user-reachable ones raise ValueError; unsupported-feature ones in the shard byte getter/setter raise ValueError/NotImplementedError; impossible-state ones (a None shard index, a missing sync transform) raise RuntimeError.

src/zarr/testing/ is excluded from S101 along with tests/: the store conformance suite and hypothesis strategies assert on purpose.

For reviewers

Behavior is meant to be unchanged for anyone not running under -O. Worth a second look:

• The exception types chosen for the converted asserts, especially RuntimeError for the impossible-state cases in sharding.py and codec_pipeline.py.
• The get_array_metadata restructure in array.py. It is the largest diff and the only one that moves logic around rather than swapping a line.
• The import-time version check in zarr/__init__.py now raises RuntimeError unconditionally instead of only outside -O.

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 (one test per new user-reachable error)
• Add docstrings and API docs for any new/modified user-facing classes and functions (none)
• New/modified features documented in docs/user-guide/*.md (n/a)
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

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