fix: remove duplicate useSSL key in polygon_node_list.yml (runtime crash)

package:yaml (used by the app) throws 'Duplicate mapping key' on the two
useSSL entries; PyYAML tolerated it so it slipped through. Adds a test that
parses every bundled YAML asset with package:yaml to prevent regression.

test/bundled_yaml_valid_test.dart

@@ -0,0 +1,29 @@
+import 'dart:io';
+import 'package:yaml/yaml.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+// Guards against the 1.0.2-era crash: a duplicate mapping key in a bundled
+// node list (polygon_node_list.yml had two `useSSL`). PyYAML tolerates it,
+// but the app parses with package:yaml, which throws "Duplicate mapping key"
+// at runtime. Every bundled *.yml the app can loadYaml() must parse cleanly.
+void main() {
+  test('all bundled node-list YAML assets parse under package:yaml', () {
+    final dir = Directory('assets');
+    final ymls = dir
+        .listSync(recursive: true)
+        .whereType<File>()
+        .where((f) => f.path.endsWith('.yml') || f.path.endsWith('.yaml'))
+        .toList();
+    expect(ymls, isNotEmpty, reason: 'no YAML assets found — wrong CWD?');
+
+    final failures = <String>[];
+    for (final f in ymls) {
+      try {
+        loadYaml(f.readAsStringSync());
+      } catch (e) {
+        failures.add('${f.path}: $e');
+      }
+    }
+    expect(failures, isEmpty, reason: 'YAML assets rejected by package:yaml:\n${failures.join('\n')}');
+  });
+}