perf: improve AI prompt cache reuse

kouyu_english/lib/core/ai_capabilities/answer_evaluation_capability.dart

@@ -9,7 +9,7 @@ class AiAnswerEvaluationCapability {
 
   static const descriptor = AiCapabilityDescriptor(
     id: 'answer-evaluation',
-    promptVersion: 1,
+    promptVersion: 2,
     outputContract: 'WritingAiFeedback',
   );
 

kouyu_english/lib/core/ai_capabilities/dialogue_coach_capability.dart

@@ -9,7 +9,7 @@ class AiDialogueCoachCapability {
 
   static const descriptor = AiCapabilityDescriptor(
     id: 'dialogue-coach',
-    promptVersion: 2,
+    promptVersion: 3,
     outputContract: 'DialogueAiResponse or DialogueAiIntervention',
   );
 

kouyu_english/lib/core/ai_capabilities/lexicon_explanation_capability.dart

@@ -9,7 +9,7 @@ class AiLexiconExplanationCapability {
 
   static const descriptor = AiCapabilityDescriptor(
     id: 'lexicon-explanation',
-    promptVersion: 1,
+    promptVersion: 2,
     outputContract: 'Temporary definition or SentenceAnalysisResult',
   );
 

kouyu_english/lib/core/ai_capabilities/review_generation_capability.dart

@@ -9,7 +9,7 @@ class AiReviewGenerationCapability {
 
   static const descriptor = AiCapabilityDescriptor(
     id: 'review-generation',
-    promptVersion: 1,
+    promptVersion: 2,
     outputContract: 'GeneratedReviewVariant or audited GeneratedLesson',
   );