Reduce the number of channels created for each node by 50% (#4064)

- Used to be 2 channels per node, it is now one per node, which is the
minimum
- Now both hard edges, conditional edges, entrypoint and conditional
entrypoint all use the same channel to trigger a node

libs/prebuilt/tests/test_react_agent.py

@@ -97,7 +97,6 @@ def test_no_prompt(
                 _AnyIdHumanMessage(content="hi?"),
                 AIMessage(content="hi?", id="0"),
             ],
-            "agent": "agent",
         }
         assert saved.metadata == {
             "parents": {},
@@ -129,7 +128,6 @@ async def test_no_prompt_async(checkpointer_name: str) -> None:
                     _AnyIdHumanMessage(content="hi?"),
                     AIMessage(content="hi?", id="0"),
                 ],
-                "agent": "agent",
             }
             assert saved.metadata == {
                 "parents": {},