feat: add JSONSerializable protocol with to_json on ArrayV3Metadata

Introduces a generic `JSONSerializable[T_co]` protocol in
`zarr.abc.serializable` parameterized on the JSON output type. Adds a
`to_json` method to `ArrayV3Metadata` that returns an
`ArrayMetadataJSON_V3` payload, demonstrating the protocol.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: d-v-b

changes/3799.feature.md

@@ -0,0 +1 @@
+Added a `JSONSerializable` protocol in `zarr.abc.serializable`, parameterized on the type returned by `to_json`. `ArrayV3Metadata` now implements this protocol via a `to_json` method that returns an `ArrayMetadataJSON_V3` typed dictionary.

src/zarr/abc/serializable.py

@@ -0,0 +1,9 @@
+from typing import Protocol
+
+
+class JSONSerializable[T_co](Protocol):
+    def to_json(self) -> T_co:
+        """
+        Serialize to a JSON-compatible Python object.
+        """
+        ...

src/zarr/core/metadata/v3.py

@@ -676,6 +676,12 @@ def to_dict(self) -> dict[str, JSON]:
             out_dict["data_type"] = dtype_meta.to_json(zarr_format=3)  # type: ignore[unreachable]
         return out_dict
 
+    def to_json(self) -> ArrayMetadataJSON_V3:
+        """
+        Serialize this array metadata to a JSON-compatible Python object.
+        """
+        return cast(ArrayMetadataJSON_V3, self.to_dict())
+
     def update_shape(self, shape: tuple[int, ...]) -> Self:
         chunk_grid = self.chunk_grid
         if isinstance(chunk_grid, RectilinearChunkGridMetadata):

tests/test_abc/test_serializable.py

@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from zarr.core.dtype.npy.int import UInt8
+from zarr.core.metadata.v3 import ArrayV3Metadata
+
+if TYPE_CHECKING:
+    from zarr.abc.serializable import JSONSerializable
+    from zarr.core.metadata.v3 import ArrayMetadataJSON_V3
+
+
+def test_array_v3_metadata_to_json() -> None:
+    """
+    ArrayV3Metadata satisfies the JSONSerializable protocol parameterized
+    on its JSON output type, and ``to_json`` returns the same payload as
+    ``to_dict``.
+    """
+    metadata = ArrayV3Metadata(
+        shape=(10,),
+        data_type=UInt8(),
+        chunk_grid={"name": "regular", "configuration": {"chunk_shape": (10,)}},
+        chunk_key_encoding={"name": "default", "configuration": {"separator": "/"}},
+        fill_value=0,
+        codecs=({"name": "bytes", "configuration": {"endian": "little"}},),
+        attributes={},
+        dimension_names=None,
+    )
+    serializable: JSONSerializable[ArrayMetadataJSON_V3] = metadata
+    assert serializable.to_json() == metadata.to_dict()