feat: #3548884 SourceValueItem field synchronized translations

christian.wiedemannrequested to merge
issue/ui_patterns-3548884:3548884-trans-make-sourcevalueitem-v2 into 2.0.x

Squashed rebase of 3548884-trans-make-sourcevalueitem onto 2.0.x, adapted to the FieldItemBase-based source field model from #3584856:

  • SourceValueItem now extends FieldItemBase (upstream model) while keeping translation extraction (preSave), read-time merge (getValue) and the synchronized_translation field setting.
  • SourceValueList extends FieldItemList instead of MapFieldItemList.
  • ComponentTree infrastructure (SourceTree, Traverser, TranslationExtractor, TranslationApplier) and kernel tests carried over unchanged.

Known pre-existing failure: SourceTreeTest::testApplyTranslationsWithList contradicts the earlier drop of list-shape auto-normalize and is slated for removal in the upcoming storage redesign.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

Closes #3548884

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