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-sqlite/tests/test_sqlite.py

@@ -72,7 +72,6 @@ class TestSqliteSaver:
             checkpoint = saver.get_tuple(config)
             assert checkpoint is not None and checkpoint.metadata == {
                 **self.metadata_2,
-                "thread_id": "thread-2",
                 "run_id": "my_run_id",
             }
 
@@ -95,18 +94,11 @@ class TestSqliteSaver:
 
             search_results_1 = list(saver.list(None, filter=query_1))
             assert len(search_results_1) == 1
-            assert search_results_1[0].metadata == {
-                "thread_id": "thread-1",
-                "thread_ts": "1",
-                **self.metadata_1,
-            }
+            assert search_results_1[0].metadata == self.metadata_1
 
             search_results_2 = list(saver.list(None, filter=query_2))
             assert len(search_results_2) == 1
-            assert search_results_2[0].metadata == {
-                "thread_id": "thread-2",
-                **self.metadata_2,
-            }
+            assert search_results_2[0].metadata == self.metadata_2
 
             search_results_3 = list(saver.list(None, filter=query_3))
             assert len(search_results_3) == 3