Avoid orphaned HistoryItemContent rows during duplicate handling

[fitchmultz]

Fixes #1368

Summary

• keep the duplicate-handling fix focused on History.add(_:)
• remove the broader one-time orphan cleanup migration and helper
• add focused regression coverage for both unsaved and already-persisted duplicate items

Details

The original change fixed the unsaved duplicate path, but it also introduced a broader cleanup migration that went beyond the narrow bugfix.

This version keeps the smallest direct fix and also covers the directly attached saved-duplicate case that surfaced while adding regression tests. In both cases, duplicate collapse should leave one history item and no orphaned HistoryItemContent rows.

Testing

xcodebuild test -project Maccy.xcodeproj -scheme Maccy -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY='' -only-testing:MaccyTests/HistoryTests/testAddingDuplicateUnsavedItemDoesNotLeaveOrphanedContentRows
xcodebuild test -project Maccy.xcodeproj -scheme Maccy -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY='' -only-testing:MaccyTests/HistoryTests/testAddingDuplicateSavedItemStillCollapsesWithoutOrphanedContentRows