fix: add missing aliases() method and improve regression test

- Add aliases() implementation to SparkDateDiff to properly expose 'datediff' alias
- Update test to use checkSparkAnswerAndOperator for complete verification
- Use fixed date instead of current_date() for reproducible testing
- Addresses PR review comments

Author: vigneshsiva11

native/spark-expr/src/datetime_funcs/date_diff.rs

@@ -68,6 +68,10 @@ impl ScalarUDFImpl for SparkDateDiff {
         Ok(DataType::Int32)
     }
 
+    fn aliases(&self) -> &[String] {
+        &self.aliases
+    }
+
     fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
         let [end_date, start_date] = take_function_args(self.name(), args.args)?;
 

spark/src/test/scala/org/apache/spark/sql/comet/ParquetDatetimeRebaseSuite.scala

@@ -133,12 +133,12 @@ abstract class ParquetDatetimeRebaseSuite extends CometTestBase {
 
         val readDf = spark.read.parquet(path.getAbsolutePath)
 
-        val result = readDf
-          .selectExpr("datediff(current_date(), ts) as diff")
-          .collect()
+        // Use a fixed date for reproducible testing
+        val resultDf = readDf
+          .selectExpr("datediff(date '2024-01-10', ts) as diff")
 
-        // Just verify it executes correctly (no CometNativeException)
-        assert(result.length == 2)
+        // Verify result correctness and plan
+        checkSparkAnswerAndOperator(resultDf)
       }
     }
   }