fix exception and add exception test

Author: lohanidamodar

src/SDK/Language/Dart.php

@@ -426,6 +426,11 @@ public function getFiles(): array
                 'destination'   => '/test/src/enums_test.dart',
                 'template'      => 'dart/test/src/enums_test.dart.twig',
             ],
+            [
+                'scope'         => 'default',
+                'destination'   => '/test/src/exception_test.dart',
+                'template'      => 'dart/test/src/exception_test.dart.twig',
+            ],
             [
                 'scope'         => 'default',
                 'destination'   => '/test/src/response_test.dart',

src/SDK/Language/Flutter.php

@@ -280,6 +280,11 @@ public function getFiles(): array
                 'destination'   => '/test/src/enums_test.dart',
                 'template'      => 'dart/test/src/enums_test.dart.twig',
             ],
+            [
+                'scope'         => 'default',
+                'destination'   => '/test/src/exception_test.dart',
+                'template'      => 'dart/test/src/exception_test.dart.twig',
+            ],
             [
                 'scope'         => 'default',
                 'destination'   => '/test/src/response_test.dart',

templates/dart/lib/src/exception.dart.twig

@@ -8,7 +8,7 @@ class {{spec.title | caseUcfirst}}Exception implements Exception {
 
   @override
   String toString() {
-    if (message == null) return "{{spec.title | caseUcfirst}}Exception";
-    return "{{spec.title | caseUcfirst}}Exception: $type, $message (${code ?? 0})";
+    if (message == null || message == "") return "{{spec.title | caseUcfirst}}Exception";
+    return "{{spec.title | caseUcfirst}}Exception: ${type ?? ''}, $message (${code ?? 0})";
   }
 }

templates/dart/test/src/exception_test.dart.twig

@@ -0,0 +1,17 @@
+import 'package:{{lang.params.packageName}}/src/exception.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('{{spec.title | caseUcfirst}}Exception', () {
+    test('toString should return correct string representation', () {
+      final exception1 = {{spec.title | caseUcfirst}}Exception();
+      expect(exception1.toString(), equals('{{spec.title | caseUcfirst}}Exception'));
+
+      final exception2 = {{spec.title | caseUcfirst}}Exception('Some error message');
+      expect(exception2.toString(), equals('{{spec.title | caseUcfirst}}Exception: , Some error message (0)'));
+
+      final exception3 = {{spec.title | caseUcfirst}}Exception('Invalid request', 400, 'ValidationError');
+      expect(exception3.toString(), equals('{{spec.title | caseUcfirst}}Exception: ValidationError, Invalid request (400)'));
+    });
+  });
+}