Extract slow path function for schemaFor

schemaFor is a very hot method, called for every message we process. We only fall through to creating the schema the first time for each class, thereafter, the hot path is just getting from the ConcurrentHashMap.

Avoid R8 inlining the slow (less common) path of schemaFor. The inlining blows up the stack frame size and slows down the hot path.

PiperOrigin-RevId: 874384247