fix: Make sum on strings error in group_by context

Co-authored-by: Claude <[email protected]>

Author: wtn

crates/polars-core/src/series/implementations/string.rs

@@ -69,6 +69,11 @@ impl private::PrivateSeries for SeriesWrap<StringChunked> {
         self.0.agg_max(groups)
     }
 
+    #[cfg(feature = "algorithm_group_by")]
+    unsafe fn agg_sum(&self, _groups: &GroupsType) -> Series {
+        invalid_operation_panic!(agg_sum, self)
+    }
+
     fn subtract(&self, rhs: &Series) -> PolarsResult<Series> {
         NumOpsDispatch::subtract(&self.0, rhs)
     }

py-polars/tests/unit/operations/test_group_by.py

@@ -12,7 +12,10 @@
 import polars as pl
 import polars.selectors as cs
 from polars import Expr
-from polars.exceptions import ColumnNotFoundError
+from polars.exceptions import (
+    ColumnNotFoundError,
+    InvalidOperationError,
+)
 from polars.meta import get_index_type
 from polars.testing import assert_frame_equal, assert_series_equal
 from polars.testing.parametric import column, dataframes, series
@@ -2724,3 +2727,10 @@ def test_agg_first_last_non_null_25405() -> None:
         }
     )
     assert_frame_equal(result.collect(), expected)
+
+
+def test_group_by_sum_on_strings_should_error_24659() -> None:
+    with pytest.raises(
+        InvalidOperationError, match=r"sum.*operation not supported for dtype.*str"
+    ):
+        pl.DataFrame({"str": ["a", "b"]}).group_by(1).agg(pl.col.str.sum())