Reduce extraneous keys in checkpoint.metadata

- Leave it up to each checkpointer implementation to decide whether to merge in configurable/metadata (previously PregelLoop would do some of this always)
- Never copy over internal langgraph keys into checkpoint.metadata (these are redundant/misleading to include)

libs/prebuilt/tests/test_react_agent.py

@@ -91,7 +91,6 @@ def test_no_prompt(sync_checkpointer: BaseCheckpointSaver, version: str) -> None
         "parents": {},
         "source": "loop",
         "step": 1,
-        "thread_id": "123",
     }
     assert saved.pending_writes == []
 
@@ -118,7 +117,6 @@ async def test_no_prompt_async(async_checkpointer: BaseCheckpointSaver) -> None:
         "parents": {},
         "source": "loop",
         "step": 1,
-        "thread_id": "123",
     }
     assert saved.pending_writes == []