[WIP] Coercing to Unicode

[Copilot]

• Understand the issue with numeric string dict keys
• Reproduce the issue with a test case
• Compare the handling in _item_added vs _item_removed
• Fix the _item_added method to use similar type checking as _item_removed
• Verify the fix with tests
• Document the changes

The issue occurs when handling dictionary keys that are numeric strings (like '1'). The key property in the PatchOperation class tries to convert these to integers, which confuses the logic in _item_added when determining if it should update array indices.

The problem is that the original code checks if type(op.key) == int and type(key) == int: to determine if it's handling arrays, but this can incorrectly match dictionary keys that are numeric strings because they get converted to integers.

This patch makes the _item_added method more robust by checking the actual data structure type (like what's already done in _item_removed) instead of relying on key type checking. This ensures that numeric string dictionary keys (like '1') are treated correctly as dictionary keys rather than array indices, preventing the "coercing to Unicode" error.

Fixes #97.

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