Fix behaviour of interrupt_before

- checkpoints are now copied before being mutated
- compiled graph and compiled state graph no longer need to override get_state/update_state
- pregel class now natively supports interrupt before/after

tests/memory_assert.py

@@ -0,0 +1,19 @@
+from langchain_core.pydantic_v1 import Field
+
+from langgraph.checkpoint.base import Checkpoint, CheckpointAt, copy_checkpoint
+from langgraph.checkpoint.memory import MemorySaver
+
+
+class MemorySaverAssertImmutable(MemorySaver):
+    storage_for_copies: dict[str, Checkpoint] = Field(default_factory=dict)
+
+    at = CheckpointAt.END_OF_STEP
+
+    def put(self, config: dict, checkpoint: dict) -> 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)
+        # call super to write checkpoint
+        super().put(config, checkpoint)