feat: set default reasoning effort to low for all AI requests

kouyu_english/lib/core/ai_config.dart

@@ -7,6 +7,7 @@ class AiConfigFile {
     required this.provider,
     required this.endpoint,
     required this.model,
+    this.reasoningEffort = 'low',
     this.apiKey,
     this.description,
   });
@@ -14,6 +15,7 @@ class AiConfigFile {
   final AiProviderType provider;
   final String endpoint;
   final String model;
+  final String reasoningEffort;
   final String? apiKey;
   final String? description;
 
@@ -25,10 +27,15 @@ class AiConfigFile {
       (p) => p.name.toLowerCase() == providerStr.toLowerCase(),
       orElse: () => AiProviderType.compatible,
     );
+    final effort = (json['reasoningEffort'] as String? ??
+            json['reasoning_effort'] as String? ??
+            'low')
+        .trim();
     return AiConfigFile(
       provider: provider,
       endpoint: (json['endpoint'] as String? ?? '').trim(),
       model: (json['model'] as String? ?? '').trim(),
+      reasoningEffort: effort.isEmpty ? 'low' : effort,
       apiKey: json['apiKey'] as String?,
       description: json['description'] as String?,
     );
@@ -43,6 +50,7 @@ class AiConfigFile {
     'provider': provider.name,
     'endpoint': endpoint,
     'model': model,
+    'reasoningEffort': reasoningEffort,
     if (apiKey != null) 'apiKey': apiKey,
     if (description != null) 'description': description,
   };
@@ -62,12 +70,14 @@ class AiConfigFile {
     AiProviderType? provider,
     String? endpoint,
     String? model,
+    String? reasoningEffort,
     String? apiKey,
     String? description,
   }) => AiConfigFile(
     provider: provider ?? this.provider,
     endpoint: endpoint ?? this.endpoint,
     model: model ?? this.model,
+    reasoningEffort: reasoningEffort ?? this.reasoningEffort,
     apiKey: apiKey ?? this.apiKey,
     description: description ?? this.description,
   );