Add test to confirm double casting behaviour

Author: shivbhatia10

datafusion/sqllogictest/test_files/spark/math/isnan.slt

@@ -70,3 +70,16 @@ true
 false
 false
 false
+
+# Spark's isnan accepts any numeric type by implicitly casting to double, so
+# integers, bigints, and decimals all return false (they can never be NaN).
+# This test guards against the signature being accidentally narrowed in a way
+# that would diverge from Spark.
+query BBBB
+SELECT
+  isnan(1) AS int_lit,
+  isnan(CAST(1 AS BIGINT)) AS bigint,
+  isnan(CAST(1 AS DECIMAL(10, 2))) AS dec,
+  isnan(CAST(NULL AS INT)) AS null_int;
+----
+false false false false