refactor: remove request_ctx from lowlevel public API, fix experimental docs example

Author: maxisbey

docs/experimental/index.md

@@ -27,13 +27,9 @@ Tasks are useful for:
 Experimental features are accessed via the `.experimental` property:
 
 ```python
-# Server-side: register a custom task handler
-server = Server(
-    name="my-server",
-    handlers=[
-        RequestHandler("tasks/get", handler=handle_get_task),
-    ],
-)
+# Server-side: enable task support (auto-registers default handlers)
+server = Server(name="my-server")
+server.experimental.enable_tasks()
 
 # Client-side
 result = await session.experimental.call_tool_as_task("tool_name", {"arg": "value"})

src/mcp/server/lowlevel/__init__.py

@@ -1,4 +1,4 @@
 from .handler import Handler, NotificationHandler, RequestHandler
-from .server import NotificationOptions, Server, request_ctx
+from .server import NotificationOptions, Server
 
 __all__ = ["Handler", "NotificationHandler", "NotificationOptions", "RequestHandler", "Server"]