Have the stream task return the accumulated string result

Author: cpsievert

shiny/ui/_markdown_stream.py

@@ -153,10 +153,15 @@ async def stream(
         async def _task():
             if clear:
                 await self._send_content_message("", "replace")
+
+            result = ""
             async with self._streaming_dot():
                 async for c in content:
+                    result += c
                     await self._send_content_message(c, "append")
 
+            return result
+
         _task()
 
         # Since the task runs in the background (outside/beyond the current context,