Refactor to composable KayaMixin architecture

Replace wrapper-based SessionMiddleware/OIDCApp with KayaMixin subclasses
applied via KayaApp(mixins=[...]). Mixins hook into handle_request and
handle_websocket via before/after hooks, so both ASGI and RSGI keep working.
Mixin dependencies are applied automatically and deduplicated.

example/session.py

@@ -1,7 +1,7 @@
 from kaya.core import HttpContext, KayaApp
-from kaya.session import InMemorySessionStore, SessionMiddleware
+from kaya.session import InMemorySessionStore, SessionMixin
 
-app = SessionMiddleware(KayaApp(), InMemorySessionStore())
+app = KayaApp(mixins=[SessionMixin(InMemorySessionStore())])
 
 
 @app.GET('/')