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/checkpoint-postgres/tests/test_async.py

@@ -228,7 +228,6 @@ async def test_combined_metadata(saver_name: str, test_data) -> None:
         checkpoint = await saver.aget_tuple(config)
         assert checkpoint.metadata == {
             **metadata,
-            "thread_id": "thread-2",
             "run_id": "my_run_id",
         }
 

libs/checkpoint-postgres/tests/test_sync.py

@@ -210,7 +210,6 @@ def test_combined_metadata(saver_name: str, test_data) -> None:
         checkpoint = saver.get_tuple(config)
         assert checkpoint.metadata == {
             **metadata,
-            "thread_id": "thread-2",
             "run_id": "my_run_id",
         }