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.txt

@@ -90,10 +90,13 @@ file:./packages/kaya-core
     # via
     #   -r requirements-dev.in
     #   kaya-oidc
+    #   kaya-openapi
     #   kaya-rsgi
     #   kaya-session
 file:./packages/kaya-oidc
     # via -r requirements-dev.in
+file:./packages/kaya-openapi
+    # via -r requirements-dev.in
 file:./packages/kaya-rsgi
     # via -r requirements-dev.in
 file:./packages/kaya-session