perf: improve AI prompt cache reuse

kouyu_english/test/ai_capabilities_test.dart

@@ -26,6 +26,16 @@ void main() {
         ),
       ),
     );
+    expect(
+      {for (final item in descriptors) item.id: item.promptVersion},
+      {
+        'speech-transcription': 1,
+        'lexicon-explanation': 2,
+        'dialogue-coach': 3,
+        'answer-evaluation': 2,
+        'review-generation': 2,
+      },
+    );
   });
 
   test(