docs(zarr-metadata): say that to_json returns arrays as tuples - #4608

The to_json docstrings and the models guide now say what to_json returns: a copy in which each JSON array is a tuple, as the package's JSON types spell an array, so it does not compare equal to what json.loads returns. They point to comparing models instead. The guide had said to_json returns plain containers.

A downstream user (refkv) reported that ZarrV3ArrayMetadata.from_json(doc).to_json() == doc is false and that isinstance(x, list) checks on the result fail. Tuples are deliberate: every JSON TypedDict in the package declares arrays as tuple[...], and to_json returns what its type declares, as pydantic's model_dump() keeps tuple fields as tuples. The docs were what was wrong.

• The six model to_json docstrings (v3 array, group, consolidated metadata; v2 array, group, .zmetadata) say each array is a tuple and each object a new dict, and point to from_json(document) == model for comparison.
• docs/models.md replaces "to_json returns plain containers" with the same explanation, noting that model comparison also treats an omitted default as equal to the default written out.

The agent rebased the branch onto main after #4605 split the landing page into guide pages; the sentence it corrects moved from docs/index.md to docs/models.md. just check in packages/zarr-metadata passes.