fix(client): copy dict handler result before stamping defaults

_handle_chat's single-shot dict path did payload = result then
setdefault'd text='' and finish_reason='stop' onto the same dict. A
chat_handler that returns a class-attribute / cached response dict had
those defaults baked into its template on every call, surprising the
caller and bleeding state across invocations.

Author: Zawwarsami16

zhub/client.py

@@ -408,7 +408,9 @@ async def _handle_chat(pub: ZhubPublication, ws, env: Envelope) -> None:
         if isinstance(result, str):
             payload = {"text": result, "finish_reason": "stop"}
         elif isinstance(result, dict):
-            payload = result
+            # Copy first — setdefault on the handler-supplied dict would mutate
+            # a caller's template / cached response by adding text:"" + finish_reason:"stop".
+            payload = dict(result)
             payload.setdefault("finish_reason", "stop")
             payload.setdefault("text", "")
         else: