[MINOR][VL] Re-enable stale ignored atan2 test in MathFunctionsValidateSuite

The atan2 test was marked ignore after a result mismatch (PR #1689, 2023)
caused by Velox using std::atan2 directly. That was fixed upstream in
oap-project/velox#263 which added sparksql::Atan2Function that mirrors
Spark's Math.atan2(y + 0.0, x + 0.0) behaviour to normalise -0 inputs.
The ignore was never removed after the fix landed.

Also promotes MathFunctionsValidateSuite from abstract class to class
(fixing the RAS-removal oversight from PR #11756, the same fix already
applied to DateFunctionsValidateSuite) and disables ANSI mode for Spark 4,
consistent with the approach in PR #12158 which adds further tests to this
same suite.

Author: brijrajk

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

@@ -66,7 +66,15 @@ class MathFunctionsValidateSuiteAnsiOn extends FunctionsValidateSuite {
   }
 }
 
-abstract class MathFunctionsValidateSuite extends FunctionsValidateSuite {
+class MathFunctionsValidateSuite extends FunctionsValidateSuite {
+
+  // Disable ANSI mode: Spark 4 enables it by default, which wraps math functions
+  // in ANSI check nodes and prevents ProjectExecTransformer from being the top-level
+  // plan node. ANSI-specific behaviour is tested in MathFunctionsValidateSuiteAnsiOn.
+  override protected def sparkConf: SparkConf = {
+    super.sparkConf
+      .set(SQLConf.ANSI_ENABLED.key, "false")
+  }
 
   disableFallbackCheck
   import testImplicits._
@@ -96,7 +104,7 @@ abstract class MathFunctionsValidateSuite extends FunctionsValidateSuite {
     }
   }
 
-  ignore("atan2") {
+  test("atan2") {
     runQueryAndCompare("SELECT atan2(double_field1, 0) from datatab limit 1") {
       checkGlutenPlan[ProjectExecTransformer]
     }