fix: fix _initialize_fresh_execution types for Haystack 2.28 (#469)

Author: anakin87

haystack_experimental/components/agents/agent.py

@@ -178,7 +178,7 @@ def __init__(  # noqa: PLR0913
 
     def _initialize_fresh_execution(
         self,
-        messages: list[ChatMessage] | None,
+        messages: list[ChatMessage],
         streaming_callback: StreamingCallbackT | None,
         requires_async: bool,
         *,