AX Agent 실행 선택 분기를 엔진으로 이관

- AxAgentExecutionEngine에 ExecutePreparedAsync를 추가해 AgentLoop와 일반 LLM 호출 선택을 엔진이 담당하도록 정리함

- SendMessageAsync와 SendRegenerateAsync가 공통 실행 진입점을 사용하도록 바꿔 창 코드의 중복 분기를 줄임

- 검증: dotnet build src/AxCopilot/AxCopilot.csproj -c Release -v minimal -p:OutputPath=bin\\verify\\ -p:IntermediateOutputPath=obj\\verify\\ 경고 0 / 오류 0

src/AxCopilot/Services/Agent/AxAgentExecutionEngine.cs

@@ -74,6 +74,17 @@ public sealed class AxAgentExecutionEngine
         return new PreparedExecution(mode, promptStack, preparedTurn.Messages);
     }
 
+    public Task<string> ExecutePreparedAsync(
+        PreparedExecution prepared,
+        Func<IReadOnlyList<ChatMessage>, CancellationToken, Task<string>> agentLoopRunner,
+        Func<IReadOnlyList<ChatMessage>, CancellationToken, Task<string>> llmRunner,
+        CancellationToken cancellationToken)
+    {
+        return prepared.Mode.UseAgentLoop
+            ? agentLoopRunner(prepared.Messages, cancellationToken)
+            : llmRunner(prepared.Messages, cancellationToken);
+    }
+
     public PreparedTurn PrepareTurn(
         ChatConversation conversation,
         IEnumerable<string?> systemPrompts,