chore(ci): 添加网络诊断步骤以排查NuGet恢复问题

添加了网络诊断步骤，包括DNS测试、curl连接测试和代理环境检查，
以便更好地排查NuGet包恢复时的网络连接问题。

.gitea/workflows/dotnet.yml

@@ -19,6 +19,17 @@ jobs:
         with:
           dotnet-version: 8.0.x
       
+      - name: Diagnose network
+        run: |
+          echo "=== DNS test ==="
+          nslookup api.nuget.org || dig api.nuget.org || echo "DNS failed"
+          echo "=== Curl test (nuget.org) ==="
+          curl -v --connect-timeout 10 https://api.nuget.org/v3/index.json 2>&1 || echo "curl failed"
+          echo "=== Curl test (huawei mirror) ==="
+          curl -v --connect-timeout 10 https://repo.huaweicloud.com/repository/nuget/v3/index.json 2>&1 || echo "curl failed"
+          echo "=== Proxy env ==="
+          env | grep -i proxy || echo "no proxy set"
+      
       - name: Restore dependencies
         run: dotnet restore --source "https://repo.huaweicloud.com/repository/nuget/v3/index.json"