Add delete_thread method to Checkpointer class (#4328)

- Deletes all data associated with a thread_id
- Implemented in InMemory, Sqlite and Postgres checkpointers

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

libs/checkpoint-sqlite/langgraph/checkpoint/sqlite/__init__.py

@@ -464,6 +464,25 @@ class SqliteSaver(BaseCheckpointSaver[str]):
                 ],
             )
 
+    def delete_thread(self, thread_id: str) -> None:
+        """Delete all checkpoints and writes associated with a thread ID.
+
+        Args:
+            thread_id (str): The thread ID to delete.
+
+        Returns:
+            None
+        """
+        with self.cursor() as cur:
+            cur.execute(
+                "DELETE FROM checkpoints WHERE thread_id = ?",
+                (str(thread_id),),
+            )
+            cur.execute(
+                "DELETE FROM writes WHERE thread_id = ?",
+                (str(thread_id),),
+            )
+
     async def aget_tuple(self, config: RunnableConfig) -> Optional[CheckpointTuple]:
         """Get a checkpoint tuple from the database asynchronously.