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

example/openapi.py

@@ -0,0 +1,32 @@
+from kaya.core import HttpContext, KayaApp
+from kaya.openapi import OpenAPIMixin, operation
+
+app = KayaApp(mixins=[OpenAPIMixin(
+    title='Greeting API',
+    version='1.0.0',
+    description='Example API documented with kaya-openapi',
+)])
+
+
+@app.GET('/hello')
+async def hello(ctx: HttpContext) -> None:
+    """Say hello to the world."""
+    await ctx.send_str(200, 'Hello World')
+
+
+@app.GET('/hello/${name}')
+@operation(summary='Greet someone',
+           tags=['greetings'],
+           responses={200: {'description': 'A personalized greeting'}})
+async def hello_name(ctx: HttpContext, name: str) -> None:
+    await ctx.send_str(200, f'Hello {name}')
+
+
+@app.GET('/square/${x:int}')
+@operation(summary='Compute the square of a number', tags=['math'])
+async def square(ctx: HttpContext, x: int) -> None:
+    await ctx.send_str(200, str(x * x))
+
+# serve with an ASGI/RSGI server, e.g.:
+#   granian --interface rsgi example.openapi:app
+# then open http://localhost:8000/docs to browse the API