"To property with serialized backing field" context action should be available inside serializable classes

The "To property with serialized backing field" context action should be available on auto properties inside a class that is marked as `[Serializable]`. Currently, it is only available on Unity derived types such as `MonoBehaviour` and `ScriptableObject`

The issue names no files, tests, or implementation entry points. Locate the context action for converting auto properties to serialized backing fields, then inspect how Serializable classes are detected alongside MonoBehaviour and ScriptableObject; done means the action appears for eligible properties inside [Serializable] classes without regressing existing Unity-derived types.

Written by the indexing model from the issue text.