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/oidc.py

@@ -1,30 +1,30 @@
 import os
 
 from kaya.core import HttpContext, KayaApp
-from kaya.oidc import OIDCConfig, OIDCApp
-from kaya.session import InMemorySessionStore, SessionMiddleware
+from kaya.oidc import OIDCConfig, OIDCMixin
+from kaya.session import InMemorySessionStore, SessionMixin
 
-app = KayaApp()
-session_app = SessionMiddleware(app, InMemorySessionStore())
-
-oidc = OIDCApp(
-    session_app,
+session = SessionMixin(InMemorySessionStore())
+oidc = OIDCMixin(
     OIDCConfig(
         issuer=os.environ.get('OIDC_ISSUER', 'https://accounts.google.com'),
         client_id=os.environ.get('OIDC_CLIENT_ID', 'replace-me'),
         client_secret=os.environ.get('OIDC_CLIENT_SECRET'),
         redirect_uri=os.environ.get('OIDC_REDIRECT_URI', 'http://localhost:8000/auth/callback'),
         fetch_userinfo=True,
-    )
+    ),
+    session=session,
 )
 
+app = KayaApp(mixins=[session, oidc])
 
-@oidc.GET('/')
+
+@app.GET('/')
 async def home(ctx: HttpContext) -> None:
     await ctx.send_str(200, 'public home')
 
 
-@oidc.GET('/profile')
+@app.GET('/profile')
 @oidc.require_auth
 async def profile(ctx: HttpContext) -> None:
     user = oidc.get_user(ctx)

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('/')