"To property with serialized backing field" context action should only be allowed on serialisable types

The context action to convert an auto property to a property with a serialised backing field should only be available on properties with types that can be serialised. E.g. should not be available on `Dictionary`

Locate the context action that converts an auto property to a property with a serialized backing field, then inspect how the property's type is checked. Verify the action is unavailable for non-serializable types such as Dictionary while remaining available for serializable types.

Written by the indexing model from the issue text.