Add remote (S3) write test: default syntax

[jhughes-mw]

tZarrWrite previously had an empty createArrayRemoteDefaultSyntax stub that was never implemented — an empty MATLAB test method passes unconditionally, so it silently reported green while covering nothing. It was removed during a test cleanup.

Task

Add a real test that creates and writes a Zarr array of varying dimensions to a remote S3 location using the default syntax (zarrcreate + zarrwrite), then reads it back with zarrread and verifies the data round-trips.

Blocked on

CI-friendly remote testing. The test must not depend on live AWS credentials — it should run against a local mock S3 (e.g. moto), which requires plumbing a custom endpoint through Zarr.extractS3BucketNameAndPath -> createKVStore (the host is currently parsed but discarded). See the team's s3-local-test-plan notes.

Acceptance criteria

• Test creates + writes + reads back a remote array with default syntax
• Runs in CI with no live AWS dependency (mock S3)
• Data verified equal after round-trip