Fix #800 Add blockItemConverters

[KevinJump]

In v16 the BlockLayoutItem classes have two obsolete properties (ContentUdi, SettingsUdi) these are often null, but sometimes not, depending on how you have asked for the content item.

these being set sometimes can cause us to think the content property has changed, when it hasn't so we get false positives, and things import when they don't need to import.

adding our own custom JsonConverters for BlockListLayoutItem and BlockGridLayoutItem, means we can control exactly how these elements are handled in the json.

these converters always write the old values out as null, (as they are not used). and this way they are consitant, and also appear as they are most likely to appear in existing imports.

for v18 we will remove them so only the keys get written out.

[Copilot]

Pull Request Overview

This PR addresses issue #800 by adding custom JSON converters to handle obsolete properties in BlockLayoutItem classes that were causing false positives during import detection. The converters ensure consistent serialization by always writing obsolete properties (ContentUdi, SettingsUdi) as null.

• Adds custom JSON converters for BlockListLayoutItem and BlockGridLayoutItem
• Ensures consistent serialization of obsolete properties to prevent false change detection
• Registers the new converters in the default JSON serialization options

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
uSync.Core/Json/XElementJsonConverter.cs	Adds import for Umbraco.Cms.Core.Models.Blocks namespace
uSync.Core/Json/JsonBlockItemConverters.cs	Implements custom JSON converters for block layout items
uSync.Core/Extensions/JsonTextExtensions.cs	Registers the new converters in serialization options

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[Copilot]

Pull Request Overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

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[Copilot]

The comment has a grammatical error. 'but they will appear as null it lots of people's existing exports' should be 'but they will appear as null in lots of people's existing exports'.

Suggested change

	// we could just not write out the obsolete properties, but they will appear as null it
	// lots of people's existing exports , so we can write them out as null to keep things consistent.
	// we could just not write out the obsolete properties, but they will appear as null in
	// lots of people's existing exports, so we can write them out as null to keep things consistent.