refactor: 清理 AI 客户端死代码并统一错误处理

- 删除未使用的 OpenAiCompatibleClient，DTO 抽离到 ai_contracts.cs
- 新增 AiHttpResponseGuard 统一 LLM/VLM 的 HTTP 错误处理：
  错误响应带响应体（截断 2000 字符）、空响应与无效 JSON 显式抛 InvalidDataException
- 空 catch 补全精确异常类型（IOException/UnauthorizedAccessException/JsonException）
- 修正 Program.cs 数据库初始化注释
- 新增 AI 客户端错误处理单元测试

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

@@ -40,7 +40,7 @@ public sealed class DeepSeekClient(HttpClient http, IConfiguration config)
         httpRequest.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("text/event-stream"));
 
         using var response = await _http.SendAsync(httpRequest, HttpCompletionOption.ResponseHeadersRead, ct);
-        response.EnsureSuccessStatusCode();
+        await AiHttpResponseGuard.ThrowIfFailedAsync(response, "DeepSeek", ct);
 
         using var stream = await response.Content.ReadAsStreamAsync(ct);
         using var reader = new StreamReader(stream);
@@ -76,11 +76,9 @@ public sealed class DeepSeekClient(HttpClient http, IConfiguration config)
         var json = JsonSerializer.Serialize(request, _jsonOptions);
         var content = new StringContent(json, Encoding.UTF8, "application/json");
 
-        var response = await _http.PostAsync("chat/completions", content, ct);
-        response.EnsureSuccessStatusCode();
-
-        var body = await response.Content.ReadAsStringAsync(ct);
-        return JsonSerializer.Deserialize<ChatCompletionResponse>(body, _jsonOptions)!;
+        using var response = await _http.PostAsync("chat/completions", content, ct);
+        var body = await AiHttpResponseGuard.ReadSuccessBodyAsync(response, "DeepSeek", ct);
+        return AiHttpResponseGuard.DeserializeRequired<ChatCompletionResponse>(body, _jsonOptions, "DeepSeek");
     }
 }
 
@@ -125,13 +123,45 @@ public sealed class VisionClient(HttpClient http, IConfiguration config)
 
         var json = JsonSerializer.Serialize(request, _jsonOptions);
         var content = new StringContent(json, Encoding.UTF8, "application/json");
-        var response = await _http.PostAsync("chat/completions", content, ct);
-        if (!response.IsSuccessStatusCode)
-        {
-            var errorBody = await response.Content.ReadAsStringAsync(ct);
-            throw new HttpRequestException($"VLM API {response.StatusCode}: {errorBody}");
-        }
-        var body = await response.Content.ReadAsStringAsync(ct);
-        return JsonSerializer.Deserialize<ChatCompletionResponse>(body, _jsonOptions)!;
+        using var response = await _http.PostAsync("chat/completions", content, ct);
+        var body = await AiHttpResponseGuard.ReadSuccessBodyAsync(response, "VLM", ct);
+        return AiHttpResponseGuard.DeserializeRequired<ChatCompletionResponse>(body, _jsonOptions, "VLM");
     }
 }
+
+internal static class AiHttpResponseGuard
+{
+    private const int MaxErrorBodyLength = 2000;
+
+    public static async Task ThrowIfFailedAsync(HttpResponseMessage response, string provider, CancellationToken ct)
+    {
+        if (response.IsSuccessStatusCode) return;
+        var body = await response.Content.ReadAsStringAsync(ct);
+        throw new HttpRequestException($"{provider} API {(int)response.StatusCode} ({response.StatusCode}): {Truncate(body)}");
+    }
+
+    public static async Task<string> ReadSuccessBodyAsync(HttpResponseMessage response, string provider, CancellationToken ct)
+    {
+        await ThrowIfFailedAsync(response, provider, ct);
+        var body = await response.Content.ReadAsStringAsync(ct);
+        if (string.IsNullOrWhiteSpace(body))
+            throw new InvalidDataException($"{provider} API返回了空响应");
+        return body;
+    }
+
+    public static T DeserializeRequired<T>(string body, JsonSerializerOptions options, string provider)
+    {
+        try
+        {
+            return JsonSerializer.Deserialize<T>(body, options)
+                ?? throw new InvalidDataException($"{provider} API响应内容为空");
+        }
+        catch (JsonException ex)
+        {
+            throw new InvalidDataException($"{provider} API响应格式无效", ex);
+        }
+    }
+
+    private static string Truncate(string value) =>
+        value.Length <= MaxErrorBodyLength ? value : value[..MaxErrorBodyLength] + "...";
+}