chore: Add missing cum_sum

Author: dangotbanned

narwhals/_plan/dummy.py

@@ -233,6 +233,9 @@ def cum_max(self, *, reverse: bool = False) -> Self:
     def cum_prod(self, *, reverse: bool = False) -> Self:
         return self._from_ir(F.CumProd(reverse=reverse).to_function_expr(self._ir))
 
+    def cum_sum(self, *, reverse: bool = False) -> Self:
+        return self._from_ir(F.CumSum(reverse=reverse).to_function_expr(self._ir))
+
     def rolling_sum(
         self, window_size: int, *, min_samples: int | None = None, center: bool = False
     ) -> Self:

narwhals/_plan/functions.py

@@ -202,6 +202,7 @@ def __repr__(self) -> str:
             CumMin: "min",
             CumMax: "max",
             CumProd: "prod",
+            CumSum: "sum",
         }
         return f"cum_{m[tp]}"
 
@@ -241,6 +242,9 @@ class CumMax(CumAgg): ...
 class CumProd(CumAgg): ...
 
 
+class CumSum(CumAgg): ...
+
+
 class RollingSum(RollingWindow): ...