Add type checks for aggregation expressions in DataFrame.aggregate method

Author: kosiew

python/datafusion/dataframe.py

@@ -549,7 +549,15 @@ def aggregate(
         group_by_exprs = [
             Expr.column(e).expr if isinstance(e, str) else e.expr for e in group_by_list
         ]
-        aggs_exprs = [e.expr for e in aggs_list]
+        aggs_exprs = []
+        for agg in aggs_list:
+            if not isinstance(agg, Expr):
+                msg = (
+                    f"Expected Expr, got {type(agg).__name__}. "
+                    "Use col() or lit() to construct expressions."
+                )
+                raise TypeError(msg)
+            aggs_exprs.append(agg.expr)
         return DataFrame(self.df.aggregate(group_by_exprs, aggs_exprs))
 
     def sort(self, *exprs: Expr | SortExpr | str) -> DataFrame: