appendEvent
Appends an event to an in-memory session object and updates the session's state based on the event's state delta, if applicable.
Note: Implementations of SessionService overriding this method should call super.appendEvent(session, event) to ensure that the caller's session object is updated and kept in sync.
This method primarily modifies the passed session object in memory. Persisting these changes typically requires a separate call to an update/save method provided by the specific service implementation, or might happen implicitly depending on the implementation's design.
If the event is marked as partial (e.g., event.partial == true), it is returned directly without modifying the session state or event list. State delta keys starting with State.TEMP_PREFIX are ignored during state updates.
Return
The appended Event instance (or the original event if it was partial).