Add kaya-openapi package for automatic OpenAPI spec generation

- New packages/kaya-openapi with OpenAPIMixin, @operation decorator,
  and generate_spec() that walks the routing tree
- Enables kaya-core's Tree.register to expose the original handler
  callback as an instance attribute for metadata introspection
- Registers GET /openapi.json and GET /docs (Swagger UI) routes
- Supports  and  path parameters, docstring
  descriptions, @operation metadata, and excludes wildcard/WS routes
- Adds example/openapi.py, updates CI, README, and requirements

requirements-dev.in

@@ -4,6 +4,7 @@ kaya-session @ file:./packages/kaya-session
 kaya-session-redis @ file:./packages/kaya-session-redis
 kaya-session-memcache @ file:./packages/kaya-session-memcache
 kaya-oidc @ file:./packages/kaya-oidc
+kaya-openapi @ file:./packages/kaya-openapi
 build
 fakeredis
 mypy