fix: use Optional[RunConfig] instead of RunConfig|None as the return type so as to consistent with rest of the code

Author: ksachdeva

src/google/adk/agents/readonly_context.py

@@ -60,6 +60,6 @@ def session(self) -> Session:
     return self._invocation_context.session
 
   @property
-  def run_config(self) -> RunConfig|None:
+  def run_config(self) -> Optional[RunConfig]:
     """The run config of the current invocation. READONLY field."""
     return self._invocation_context.run_config