fix(wechat): 创建二维码 POST 改用 StringContent 避免微信 412

PostAsJsonAsync 可能走 chunked 传输缺少 Content-Length，微信网关直接返回 HTTP 412 空响应。

Co-authored-by: Cursor <cursoragent@cursor.com>

src/SimpleAdmin.Api/WeChat/Services/WeChatMpApiClient.cs

@@ -1,3 +1,4 @@
+using System.Text;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 using System.Xml.Linq;
@@ -71,9 +72,12 @@ internal sealed class WeChatMpApiClient(
             }
         };
 
-        using var response = await client.PostAsJsonAsync(
+        // 微信服务端要求 POST 带 Content-Length；PostAsJsonAsync 可能走 chunked 导致 HTTP 412
+        var json = JsonSerializer.Serialize(payload);
+        using var content = new StringContent(json, Encoding.UTF8, "application/json");
+        using var response = await client.PostAsync(
             $"https://api.weixin.qq.com/cgi-bin/qrcode/create?access_token={Uri.EscapeDataString(accessToken)}",
-            payload,
+            content,
             cancellationToken);
 
         var result = await ReadWeChatJsonAsync<WeChatQrTicketResponse>(
@@ -206,8 +210,11 @@ internal sealed class WeChatMpApiClient(
                 action,
                 (int)response.StatusCode,
                 raw);
+            var hint = response.StatusCode == System.Net.HttpStatusCode.PreconditionFailed
+                ? "（常见于 POST 未带 Content-Length 或出口 IP 未加白名单，请检查服务端 HttpClient 与公众号 IP 白名单）"
+                : string.Empty;
             throw new InvalidOperationException(
-                $"微信{action}失败 HTTP {(int)response.StatusCode} {response.ReasonPhrase}: {raw}");
+                $"微信{action}失败 HTTP {(int)response.StatusCode} {response.ReasonPhrase}: {raw}{hint}");
         }
 
         if (string.IsNullOrWhiteSpace(raw))