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

packages/kaya-core/src/kaya/core/_tree.py

@@ -186,16 +186,27 @@ class Tree:
                  callback: Callable[[Context, Unpack[Any]], Awaitable[None]],
                  recursive: bool) -> None:
         class Handler(PathHandler):
+            """PathHandler created by :meth:`Tree.register`.
+
+            The original user callback is exposed through the ``callback``
+            attribute so that extensions (e.g. ``kaya-openapi``) can inspect
+            it for metadata such as docstrings or decorator attributes.
+            """
+
+            callback: Callable[[Context, Unpack[Any]], Awaitable[None]]
 
             async def handle_request(self, ctx: Context, captured: Matches) -> None:
                 args = Maybe.of_nullable(captured.path).map(lambda it: [it]).or_else([])
-                await callback(ctx, *args, **captured.kwargs)
+                await self.callback(ctx, *args, **captured.kwargs)
 
             @property
             def recursive(self) -> bool:
                 return recursive
 
         handler = Handler()
+        # assigned as an instance attribute (not a class attribute) so that
+        # the function descriptor protocol does not turn it into a bound method
+        handler.callback = callback
         self.add((p for p in PathIterator(path)), method, handler)
 
     def find_node(self, path: Generator[str, None, None], method: HttpMethod = HttpMethod.GET) \