Fix invalid channels error message (#4357)

# PR Summary
This small PR resolves the error formatting in
`libs/langgraph/langgraph/pregel/algo.py` so the `proc.channels` will be
evaluated in the message.

Signed-off-by: Emmanuel Ferdman <emmanuelferdman@gmail.com>

libs/langgraph/langgraph/pregel/algo.py

@@ -1069,7 +1069,7 @@ def _proc_input(
             return MISSING
     else:
         raise RuntimeError(
-            "Invalid channels type, expected list or dict, got {proc.channels}"
+            f"Invalid channels type, expected list or dict, got {proc.channels}"
         )
 
     # If the process has a mapper, apply it to the value