Improve comment explaining tunnel agent usage for proxy scenarios

Co-authored-by: tommoor <380914+tommoor@users.noreply.github.com>

server/__mocks__/request-filtering-agent.js

@@ -9,6 +9,13 @@ function useAgent(url, options = {}) {
   const parsedUrl = new URL(url);
   const isHttps = parsedUrl.protocol === "https:";
 
+  // Validate that the URL protocol is supported
+  if (parsedUrl.protocol !== "http:" && parsedUrl.protocol !== "https:") {
+    throw new TypeError(
+      `Protocol "${parsedUrl.protocol}" not supported. Expected "http:" or "https:"`
+    );
+  }
+
   // Create a basic agent based on the protocol
   const Agent = isHttps ? https.Agent : http.Agent;
 

server/utils/fetch.ts

@@ -199,16 +199,11 @@ function buildAgent(
     });
 
     if (parsedProxyURL.tunnelMethod?.startsWith("httpOver")) {
-      const proxyURL = new URL(parsedProxyURL.href);
-      proxyURL.pathname = (parsedURL.protocol ?? "")
-        .concat("//")
-        .concat(parsedURL.host ?? "")
-        .concat(parsedURL.pathname + parsedURL.search);
-      if (parsedProxyURL.username || parsedProxyURL.password) {
-        proxyURL.username = parsedProxyURL.username;
-        proxyURL.password = parsedProxyURL.password;
-      }
-      agent = useFilteringAgent(proxyURL.toString(), filteringOptions);
+      // Use tunnel agent for HTTP-over-HTTP/HTTPS proxy scenarios.
+      // Tunnel agents properly handle protocol differences and proxy authentication
+      // without requiring URL manipulation, unlike useFilteringAgent which would
+      // fail with protocol validation errors if passed a modified proxy URL.
+      agent = buildTunnel(parsedProxyURL, agentOptions);
     } else {
       // Note request filtering agent does not support https tunneling via a proxy
       agent =