feat: improve AI dialogue evaluation and capability boundaries

kouyu_english/lib/features/lesson/steps/writing_step.dart

@@ -57,15 +57,16 @@ class _WritingStepState extends State<_WritingStep> {
       requestingAiFeedback = true;
       aiFeedbackError = null;
     });
-    final feedback = await AiService.instance.writingFeedback(
-      provider: widget.state.aiProvider,
-      endpoint: widget.state.aiEndpoint,
-      model: widget.state.aiModel,
-      lessonId: widget.lessonId,
-      taskPrompt: widget.activity.writingPrompt,
-      answer: widget.controller.text.trim(),
-      level: lessonLevel(widget.lessonId),
-    );
+    final feedback = await AiService.instance.capabilities.evaluation
+        .evaluateWriting(
+          provider: widget.state.aiProvider,
+          endpoint: widget.state.aiEndpoint,
+          model: widget.state.aiModel,
+          lessonId: widget.lessonId,
+          taskPrompt: widget.activity.writingPrompt,
+          answer: widget.controller.text.trim(),
+          level: lessonLevel(widget.lessonId),
+        );
     if (!mounted) return;
     setState(() {
       requestingAiFeedback = false;
@@ -88,9 +89,7 @@ class _WritingStepState extends State<_WritingStep> {
         ),
         SectionCard(
           tint: AppColors.surfaceMuted,
-          child: Text(
-            '小提示：${grammarNoteForSegment(widget.segmentId)}',
-          ),
+          child: Text('小提示：${grammarNoteForSegment(widget.segmentId)}'),
         ),
         if (widget.showHelp)
           SectionCard(