cmCTestCurl: Avoid using undocumented type for CURLOPT_PROXYTYPE values

Since upstream curl commit `1a12663d06` (CURLOPT: bump `CURLPROXY_*`
enums to `long`, drop casts, 2025-07-28), the `CURLPROXY_*` constants
are integer literals instead of `enum curl_proxytype`.  It turns out
that `curl_easy_setopt` has always expected a `long` anyway, and that
`curl_proxytype` is not documented for public use.

Fixes: #27178

Source/CTest/cmCTestCurl.h

@@ -52,7 +52,7 @@ private:
   std::vector<std::string> HttpHeaders;
   std::string HTTPProxyAuth;
   std::string HTTPProxy;
-  curl_proxytype HTTPProxyType;
+  long HTTPProxyType;
   bool UseHttp10 = false;
   bool Quiet = false;
   int TimeOutSeconds = 0;