test: Restore ParseCoreData session state between tests

Tests in this group mutate the singleton ParseCoreData().sessionId
and were not restoring it, leaking state across cases. Capture the
prior session in setUp() and restore it in tearDown() so each test
starts from a known baseline.

Addresses CodeRabbit nit on PR #1139.

Author: chadpav

packages/dart/test/src/objects/parse_user/parse_user_session_token_test.dart

@@ -14,6 +14,7 @@ void main() {
 
   group('ParseUser save()/update() — sessionToken adoption from response', () {
     late MockParseClient client;
+    String? previousSessionId;
 
     const String userObjectId = 'sess123';
     final String putPath = Uri.parse(
@@ -22,6 +23,11 @@ void main() {
 
     setUp(() {
       client = MockParseClient();
+      previousSessionId = ParseCoreData().sessionId;
+    });
+
+    tearDown(() {
+      ParseCoreData().sessionId = previousSessionId;
     });
 
     test(