perf: improve AI prompt cache reuse

kouyu_english/test/deepseek_thinking_test.dart

@@ -100,13 +100,15 @@ void main() {
     );
 
     expect(reply?.reply, 'Where are you from?');
-    expect(messages, hasLength(3));
+    expect(messages, hasLength(4));
     expect(messages![0]['role'], 'system');
-    expect(jsonDecode(messages![1]['content'] as String), {
+    expect(messages![1]['role'], 'user');
+    expect(messages![1]['content'], startsWith('[ConversationConfig]'));
+    expect(jsonDecode(messages![2]['content'] as String), {
       'reply': "Hi! What's your name?",
     });
-    expect(messages![2]['role'], 'user');
-    final last = messages![2]['content'] as String;
+    expect(messages![3]['role'], 'user');
+    final last = messages![3]['content'] as String;
     expect(last, startsWith('My name is Alex.'));
     expect(
       last,
@@ -114,28 +116,33 @@ void main() {
     );
     expect(
       last,
-      endsWith('Do not repeat any questions or greetings already asked or answered.'),
+      endsWith(
+        'Do not repeat any questions or greetings already asked or answered.',
+      ),
     );
     expect(messages![0]['content'], isNot(contains('Ask where the learner')));
   });
 
-  test('dialogue system prompt stays identical across turns', () {
+  test('dialogue system prompt stays identical across levels and lessons', () {
     final words = ['Excuse me.', 'Can you help me?'];
-    final a = AiService.dialogueSystemPrompt(
-      level: 'A1',
-      allowedLanguage: words,
-    );
-    final b = AiService.dialogueSystemPrompt(
-      level: 'A1',
-      allowedLanguage: words,
-    );
+    final a = AiService.dialogueSystemPrompt();
+    final b = AiService.dialogueSystemPrompt();
     expect(a, b);
-    expect(a, contains('CEFR A1'));
     expect(a, contains('Strictly do not repeat any question'));
-    expect(a, endsWith('Taught language: Excuse me.; Can you help me?'));
+    expect(a, isNot(contains('Excuse me.')));
+
+    final config = AiService.dialogueConversationConfig(
+      level: 'A1',
+      allowedLanguage: words,
+    );
+    expect(config, contains('A1'));
+    expect(config, contains('Excuse me.'));
     expect(
-      AiService.dialogueSystemPrompt(level: 'A0', allowedLanguage: const []),
-      contains('A0 English conversation partner'),
+      AiService.dialogueConversationConfig(
+        level: 'A0',
+        allowedLanguage: const [],
+      ),
+      contains('simple, common A0 English'),
     );
   });
 }