refactor(langgraph): simplify Send sanitization with dict comprehension

Co-authored-by: sydney-runkle <54324534+sydney-runkle@users.noreply.github.com>

libs/langgraph/langgraph/pregel/_algo.py

@@ -1121,14 +1121,10 @@ def sanitize_untracked_values_in_send(
         # Command
         return packet
 
-    sanitized_arg = dict(packet.arg)
-
-    # top level keys should be the channel names
-    to_pop = set()
-    for k, v in sanitized_arg.items():
-        if isinstance(channels.get(k), UntrackedValue):
-            to_pop.add(k)
-    for k in to_pop:
-        sanitized_arg.pop(k)
+    sanitized_arg = {
+        k: v
+        for k, v in packet.arg.items()
+        if not isinstance(channels.get(k), UntrackedValue)
+    }
 
     return Send(node=packet.node, arg=sanitized_arg)