fix: VLM 参数优化 - temperature 0.7, top_p 0.8, 指令放 system+user

- VisionAsync 新增 Temperature=0.7, TopP=0.8
- system prompt 用专业营养识别指令
- userText 用简短"请看图识别食物"配合图片
- 修复重复 prompt 导致 VLM 误读文本的 bug

backend/src/Health.Infrastructure/AI/open_ai_compatible_client.cs

@@ -164,6 +164,7 @@ public sealed class ChatCompletionRequest
     public bool Stream { get; set; }
     public int MaxTokens { get; set; } = 2048;
     public float Temperature { get; set; } = 0.7f;
+    public float? TopP { get; set; }
     public List<ToolDefinition>? Tools { get; set; }
     public string? ToolChoice { get; set; }
 }