Harden WinHTTP proxy resolution and add fallback

Program.cs

@@ -265,6 +265,12 @@ class Program
 
 static class WinHttpHelper
 {
+    const int WINHTTP_ACCESS_TYPE_NO_PROXY = 1;
+    const int WINHTTP_AUTOPROXY_AUTO_DETECT = 0x00000001;
+    const int WINHTTP_AUTOPROXY_CONFIG_URL = 0x00000002;
+    const int WINHTTP_AUTO_DETECT_TYPE_DHCP = 0x00000001;
+    const int WINHTTP_AUTO_DETECT_TYPE_DNS_A = 0x00000002;
+
     [DllImport("winhttp.dll", SetLastError = true)]
     static extern IntPtr WinHttpOpen(string agent, int accessType, string proxy, string bypass, int flags);
 
@@ -278,38 +284,57 @@ static class WinHttpHelper
     [DllImport("winhttp.dll")]
     static extern bool WinHttpCloseHandle(IntPtr handle);
 
+    [DllImport("winhttp.dll", SetLastError = true)]
+    static extern bool WinHttpGetDefaultProxyConfiguration(out WINHTTP_PROXY_INFO proxyInfo);
+
     public static Uri GetProxyForUrl(Uri url)
     {
-        IntPtr session = WinHttpOpen("proxy", 0, null, null, 0);
+        IntPtr session = WinHttpOpen("proxy", WINHTTP_ACCESS_TYPE_NO_PROXY, null, null, 0);
         if (session == IntPtr.Zero)
         {
-            Log.Error("WinHttpOpen failed");
+            Log.Error($"WinHttpOpen failed (Win32={Marshal.GetLastWin32Error()})");
             return url;
         }
 
         var options = new WINHTTP_AUTOPROXY_OPTIONS
         {
-            dwFlags = 0x00000001 | 0x00000002,
-            dwAutoDetectFlags = 0x00000001 | 0x00000002
+            dwFlags = WINHTTP_AUTOPROXY_AUTO_DETECT | WINHTTP_AUTOPROXY_CONFIG_URL,
+            dwAutoDetectFlags = WINHTTP_AUTO_DETECT_TYPE_DHCP | WINHTTP_AUTO_DETECT_TYPE_DNS_A,
+            fAutoLogonIfChallenged = true
         };
 
-        if (WinHttpGetProxyForUrl(session, url.ToString(), ref options, out var info))
+        if (WinHttpGetProxyForUrl(session, url.ToString(), ref options, out var autoInfo))
         {
-            string proxy = PtrToString(info.lpszProxy);
-
-            if (!string.IsNullOrEmpty(proxy))
+            var proxyUri = ParseProxyInfo(autoInfo);
+            if (proxyUri != null)
             {
-                var first = proxy.Split(';')[0];
                 WinHttpCloseHandle(session);
-                return new Uri(Normalize(first));
+                return proxyUri;
             }
         }
         else
         {
-            Log.Error($"WinHttpGetProxyForUrl failed for {url} (Win32={Marshal.GetLastWin32Error()})");
+            int err = Marshal.GetLastWin32Error();
+            Log.Warn($"WinHttpGetProxyForUrl failed for {url} (Win32={err})");
+        }
+
+        if (WinHttpGetDefaultProxyConfiguration(out var defaultInfo))
+        {
+            var fallbackProxy = ParseProxyInfo(defaultInfo);
+            if (fallbackProxy != null)
+            {
+                Log.Info($"Using default WinHTTP proxy {fallbackProxy.Host}:{fallbackProxy.Port} for {url}");
+                WinHttpCloseHandle(session);
+                return fallbackProxy;
+            }
+        }
+        else
+        {
+            Log.Warn($"WinHttpGetDefaultProxyConfiguration failed (Win32={Marshal.GetLastWin32Error()})");
         }
 
         WinHttpCloseHandle(session);
+        Log.Warn($"No upstream proxy resolved for {url}, fallback to direct");
         return url;
     }
 
@@ -319,11 +344,43 @@ static class WinHttpHelper
         return Marshal.PtrToStringUni(ptr);
     }
 
-    static string Normalize(string proxy)
+    static Uri ParseProxyInfo(WINHTTP_PROXY_INFO info)
     {
-        if (!proxy.StartsWith("http"))
-            return "http://" + proxy;
-        return proxy;
+        var raw = PtrToString(info.lpszProxy);
+        if (string.IsNullOrWhiteSpace(raw))
+            return null;
+
+        // Common WinHTTP formats:
+        // "proxy.local:8080"
+        // "http=proxy.local:8080;https=proxy.local:8443"
+        var token = raw.Split(';', StringSplitOptions.RemoveEmptyEntries)
+            .Select(p => p.Trim())
+            .FirstOrDefault(p => p.StartsWith("https=", StringComparison.OrdinalIgnoreCase))
+            ?? raw.Split(';', StringSplitOptions.RemoveEmptyEntries)
+                .Select(p => p.Trim())
+                .FirstOrDefault(p => p.StartsWith("http=", StringComparison.OrdinalIgnoreCase))
+            ?? raw.Split(';', StringSplitOptions.RemoveEmptyEntries)
+                .Select(p => p.Trim())
+                .FirstOrDefault();
+
+        if (string.IsNullOrWhiteSpace(token))
+            return null;
+
+        var hostPort = token.Contains('=')
+            ? token.Split('=', 2)[1].Trim()
+            : token;
+
+        var normalized = hostPort.StartsWith("http", StringComparison.OrdinalIgnoreCase)
+            ? hostPort
+            : "http://" + hostPort;
+
+        if (!Uri.TryCreate(normalized, UriKind.Absolute, out var uri))
+        {
+            Log.Warn($"Cannot parse proxy value '{raw}'");
+            return null;
+        }
+
+        return uri;
     }
 
     struct WINHTTP_AUTOPROXY_OPTIONS