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/tests/Health.Tests/ai_client_error_tests.cs

@@ -0,0 +1,50 @@
+using System.Net;
+using Health.Infrastructure.AI;
+using Microsoft.Extensions.Configuration;
+
+namespace Health.Tests;
+
+public sealed class AiClientErrorTests
+{
+    [Fact]
+    public async Task DeepSeek_ErrorResponse_IncludesStatusAndBody()
+    {
+        var client = CreateClient(HttpStatusCode.TooManyRequests, "{\"error\":\"rate limit\"}");
+
+        var error = await Assert.ThrowsAsync<HttpRequestException>(() => client.ChatAsync([]));
+
+        Assert.Contains("429", error.Message);
+        Assert.Contains("rate limit", error.Message);
+    }
+
+    [Fact]
+    public async Task DeepSeek_NullJsonResponse_ThrowsClearDataError()
+    {
+        var client = CreateClient(HttpStatusCode.OK, "null");
+
+        var error = await Assert.ThrowsAsync<InvalidDataException>(() => client.ChatAsync([]));
+
+        Assert.Contains("响应内容为空", error.Message);
+    }
+
+    private static DeepSeekClient CreateClient(HttpStatusCode statusCode, string body)
+    {
+        var http = new HttpClient(new StubHandler(statusCode, body))
+        {
+            BaseAddress = new Uri("https://example.test/"),
+        };
+        var config = new ConfigurationBuilder().AddInMemoryCollection(
+            new Dictionary<string, string?> { ["DEEPSEEK_MODEL"] = "test-model" }).Build();
+        return new DeepSeekClient(http, config);
+    }
+
+    private sealed class StubHandler(HttpStatusCode statusCode, string body) : HttpMessageHandler
+    {
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) =>
+            Task.FromResult(new HttpResponseMessage(statusCode)
+            {
+                Content = new StringContent(body),
+                RequestMessage = request,
+            });
+    }
+}