fix: add exception handling for non-YamlMap entry values and improve …

…rule retrieval process

tools/update_lint_rules/lib/src/services/lint_rule_service.dart

@@ -136,9 +136,14 @@ class LintRuleService {
 
     final rules = lintCode.entries
         .fold<List<Map<String, dynamic>>>([], (rules, entry) {
+          final entryValue = entry.value;
+          if (entryValue is! YamlMap) {
+            throw FormatException('entryValue is not YamlMap: $entryValue');
+          }
+
           final rule = {
-            nameKey: entry.value[sharedNameKey] ?? entry.key,
-            ...convertToJsonFromYaml(entry.value),
+            nameKey: entryValue[sharedNameKey] ?? entry.key,
+            ...convertToJsonFromYaml(entryValue),
           };
 
           if (rule[sharedNameKey] != null &&