refactor: Separate registry logic from ChunkGrid base class (fixes #3734)

[abishop1990]

Summary

Refactor the ChunkGrid base class to remove registry responsibility.

Changes

• Remove ChunkGrid.from_dict class method that handled dispatch to subclasses
• Create new parse_chunk_grid function to handle deserialization and dispatch
• Update ArrayV3Metadata to use parse_chunk_grid instead of ChunkGrid.from_dict

Benefits

• Separation of concerns: base class no longer has registry responsibility
• Consistency: follows the same pattern as parse_chunk_key_encoding
• Clarity: registry logic is now explicit and isolated
• Extensibility: new chunk grid types can be registered without modifying base class

Testing

• All lint checks pass
• Syntax validation passes
• Minimal surface area: only 3 files modified

[d-v-b]

#3815 (comment)

[abishop1990]

Closing: Changelog validation continues to fail despite multiple fixes. The approach needs review before re-opening. Issue #3734 remains open for future work.