V16/new trash handling

[KevinJump]

fix for #799 - Trash does not correctly import on a clean site.

the change isn't massive, we were not handling the case where the item was trashed, but it was also imported first time - this means because we never hit the internal MoveToRecycleBin call, the item Trashed state was never set to true.

This change fixes that, and makes sure the delete relation is in place so the restore knows where the item goes back to in the site.

Move's trash handling to the second pass, because when creating the relation, you need the item to have an internal id (int) and that doesn't happen until it's been saved (in the first pass).

We have tidies up trash handling, so there is less duplication between the media and content serializers.

[Copilot]

Pull Request Overview

This PR fixes trash handling for imported items by moving trash processing to the second pass and consolidating duplicate code between media and content serializers. The fix addresses issue #799 where trashed items weren't correctly imported on clean sites.

• Moves trash handling logic from first pass to second pass to ensure items have internal IDs for relation creation
• Consolidates duplicate trash handling code into the base class with abstract methods for type-specific operations
• Adds JSON converters for block layout items to handle obsolete properties causing false positives

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
MediaSerializer.cs	Removes first-pass trash handling and implements abstract methods for media-specific operations
ContentSerializer.cs	Removes duplicate trash handling code and implements abstract methods for content-specific operations
ContentSerializerBase.cs	Adds consolidated trash handling logic and abstract method definitions
JsonBlockItemConverters.cs	New converters for consistent block layout item serialization
JsonTextExtensions.cs	Registers the new block item converters
XElementJsonConverter.cs	Adds trailing newline

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