Update changelog

Author: maxrjones

changes/3802.feature.md

@@ -9,3 +9,8 @@ Rectilinear chunks can be used through:
 - **Opening existing arrays**: Arrays stored with the ``rectilinear`` chunk grid are read
   transparently via ``zarr.open`` and ``zarr.open_array``.
 - **Rectilinear sharding**: Shard boundaries can be rectilinear while inner chunks remain regular.
+
+**Breaking change**: The ``validate`` method on ``BaseCodec`` and ``CodecPipeline`` now receives
+a ``ChunkGridMetadata`` instance instead of a ``ChunkGrid`` instance for the ``chunk_grid``
+parameter. Third-party codecs that override ``validate`` and inspect the chunk grid will need to
+update their type annotations. No known downstream packages were using this parameter.