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

@@ -16,6 +16,7 @@
 // under the License.
 
 use arrow::array::{Array, Date32Array, Int32Array};
+use arrow::compute::cast;
 use arrow::compute::kernels::arity::binary;
 use arrow::datatypes::DataType;
 use datafusion::common::{utils::take_function_args, DataFusionError, Result};
@@ -68,6 +69,10 @@ 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)?;