Add history tracking to in-memory, sqlite and aiosqlite checkpointers

- Add Pregel.get_state_history and .aget_state_history methods to get history iterator
- Update checkpointer base class with new list and get_tuple methods
- Rewrite in-memory checkpointer class to track history
- Rewrite sqlite and aiosqlite checkpointers to track history
- Add new tests for history tracking

tests/memory_assert.py

@@ -1,3 +1,5 @@
+from collections import defaultdict
+
 from langchain_core.pydantic_v1 import Field
 
 from langgraph.checkpoint.base import Checkpoint, CheckpointAt, copy_checkpoint
@@ -5,15 +7,19 @@ from langgraph.checkpoint.memory import MemorySaver
 
 
 class MemorySaverAssertImmutable(MemorySaver):
-    storage_for_copies: dict[str, Checkpoint] = Field(default_factory=dict)
+    storage_for_copies: defaultdict[str, dict[str, Checkpoint]] = Field(
+        default_factory=lambda: defaultdict(dict)
+    )
 
     at = CheckpointAt.END_OF_STEP
 
-    def put(self, config: dict, checkpoint: dict) -> None:
+    def put(self, config: dict, checkpoint: Checkpoint) -> None:
         # assert checkpoint hasn't been modified since last written
         thread_id = config["configurable"]["thread_id"]
         if saved := super().get(config):
-            assert self.storage_for_copies[thread_id] == saved
-        self.storage_for_copies[thread_id] = copy_checkpoint(checkpoint)
+            assert self.storage_for_copies[thread_id][saved["ts"]] == saved
+        self.storage_for_copies[thread_id][checkpoint["ts"]] = copy_checkpoint(
+            checkpoint
+        )
         # call super to write checkpoint
         super().put(config, checkpoint)