Add validation test for CurrentTimestamp

Author: Mariam-Almesfer

backends-velox/src/test/scala/org/apache/gluten/functions/ScalarFunctionsValidateSuite.scala

@@ -1521,4 +1521,22 @@ abstract class ScalarFunctionsValidateSuite extends FunctionsValidateSuite {
         }
     }
   }
+
+  test("current_timestamp") {
+    withSQLConf("spark.sql.optimizer.excludedRules" -> "") {
+      val df = spark.sql("SELECT current_timestamp()")
+
+      val optimizedPlan = df.queryExecution.optimizedPlan.toString()
+      assert(
+        optimizedPlan.contains("Project") && !optimizedPlan.contains("CurrentTimestamp"),
+        s"Expected CurrentTimestamp to be folded to a literal, but got: $optimizedPlan"
+      )
+
+      checkFallbackOperators(df, 1)
+
+      val result = df.collect()
+      assert(result.length == 1, "Expected exactly one row")
+      assert(result(0).getTimestamp(0) != null, "Expected a non-null timestamp")
+    }
+  }
 }