Migrations: Adjust the JsonBlockValueConverter to handle conflicts with 'values' property

[lauraneto]

Fixes #20409

This pull request improves the robustness of block item data deserialization in JsonBlockValueConverter by introducing a fallback mechanism for handling legacy data schemas and resolving conflicts with the values property, which was added in #17120.
The changes ensure that data from older formats can still be deserialized without errors, while making it clear that these methods are temporary and will be removed in a future version.

Deserialization robustness and legacy schema support:

• Enhanced the DeserializeBlockItemData method to catch JsonExceptions related to the .values property and use a fallback deserialization approach for legacy data schemas.
• Added a suite of [Obsolete] methods (FallbackBlockItemDataDeserialization, DeserializeBlockItemElement, DeserializeWithRenamedValues, CreateModifiedJsonWithRenamedValues, and RestoreOriginalValuesPropertyName) to handle legacy block item data, specifically addressing cases where the values property causes deserialization conflicts. These methods rename the conflicting property temporarily and restore it after deserialization.

Testing

The reporter of the issue added a repository that contains a SQLite database that will trigger the issue, but this can also be reproduced by simply creating a v13 solution with a block list configured with a block with a property called values, adding sample content and then trying to upgrade to v15+.
This fallback logic should only really be called during the V_15_0_0.ConvertBlockListEditorProperties migration. After that, the data should be fixed and the code shouldn't trigger anymore.

[Copilot]

Pull Request Overview

This PR fixes a deserialization conflict in JsonBlockValueConverter where the values property conflicts with the internal Values collection used by the block system. The issue occurs when migrating from v13 to v15+ where legacy block data contains a user-defined property named "values".

• Adds a try-catch mechanism to detect JsonExceptions related to .values property conflicts
• Implements a fallback deserialization approach that temporarily renames conflicting properties
• Introduces robust error handling for legacy data schemas during migration

[AndyButland]

Works nicely @lauraneto - it's an edge case, but perhaps not an uncommon one, but good that we have it covered. I saw you pushed an update recently so I'll approve but not merge and leave you to do that when you are happy.

I pushed up a small unit test to verify the behaviour - this failed with the old code but passes with what you have in the PR. I also tested with the provided database.