Issue #3336551: Improve FieldItemBase class: null-safe getParent() and proxy property definitions
Closes #3336551
- Added null-safe checks for getParent() in getEntity() and getLangcode().
- Replaced direct use of $this->definition with getDataDefinition() where applicable.
- Added proxy methods getPropertyDefinitions() and getPropertyDefinition($name).
- Updated FieldItemTest to be Drupal 11 compatible, replacing getName() with getDataType().