Add focused tests for cross join and scalar subquery

Introduce two tests to identify the promotion path
behind the regression. The tests validate that the
cross join is represented as Inner Join with no
join filters and that a scalar subquery condition
is considered a candidate for join evaluation.
These changes help pinpoint the branch in
push_down_all_join affecting the transformation
of the Cross Join and Filter into Inner Join.

Author: kosiew

datafusion/optimizer/src/push_down_filter.rs

@@ -2471,6 +2471,33 @@ mod tests {
         )
     }
 
+    #[test]
+    fn cross_join_builder_uses_inner_join_with_no_join_keys() -> Result<()> {
+        let plan = LogicalPlanBuilder::from(test_table_scan()?)
+            .cross_join(test_table_scan_with_name("test1")?)?
+            .build()?;
+
+        let LogicalPlan::Join(join) = plan else {
+            panic!("expected join plan");
+        };
+
+        assert_eq!(join.join_type, JoinType::Inner);
+        assert!(join.on.is_empty());
+        assert!(join.filter.is_none());
+
+        Ok(())
+    }
+
+    #[test]
+    fn scalar_subquery_cross_join_filter_is_treated_as_join_condition_candidate(
+    ) -> Result<()> {
+        let predicate = col("s.acctbal").gt(col("__scalar_sq_1.avg_acctbal"));
+
+        assert!(can_evaluate_as_join_condition(&predicate)?);
+
+        Ok(())
+    }
+
     /// verifies that filters with the same columns are correctly placed
     #[test]
     fn filter_2_breaks_limits() -> Result<()> {