fix(security): block plaintext WebSocket connections to non-loopback addresses (#20803)

* fix(security): block plaintext WebSocket connections to non-loopback addresses

Addresses CWE-319 (Cleartext Transmission of Sensitive Information).

Previously, ws:// connections to remote hosts were allowed, exposing
both credentials and chat data to network interception. This change
blocks ALL plaintext ws:// connections to non-loopback addresses,
regardless of whether explicit credentials are configured (device
tokens may be loaded dynamically).

Security policy:
- wss:// allowed to any host
- ws:// allowed only to loopback (127.x.x.x, localhost, ::1)
- ws:// to LAN/tailnet/remote hosts now requires TLS

Changes:
- Add isSecureWebSocketUrl() validation in net.ts
- Block insecure connections in GatewayClient.start()
- Block insecure URLs in buildGatewayConnectionDetails()
- Handle malformed URLs gracefully without crashing
- Update tests to use wss:// for non-loopback URLs

Fixes #12519

* fix(test): update gateway-chat mock to preserve net.js exports

Use importOriginal to spread actual module exports and mock only
the functions needed for testing. This ensures isSecureWebSocketUrl
and other exports remain available to the code under test.

src/gateway/net.ts

@@ -396,3 +396,33 @@ export function isLoopbackHost(host: string): boolean {
   const unbracket = h.startsWith("[") && h.endsWith("]") ? h.slice(1, -1) : h;
   return isLoopbackAddress(unbracket);
 }
+
+/**
+ * Security check for WebSocket URLs (CWE-319: Cleartext Transmission of Sensitive Information).
+ *
+ * Returns true if the URL is secure for transmitting data:
+ * - wss:// (TLS) is always secure
+ * - ws:// is only secure for loopback addresses (localhost, 127.x.x.x, ::1)
+ *
+ * All other ws:// URLs are considered insecure because both credentials
+ * AND chat/conversation data would be exposed to network interception.
+ */
+export function isSecureWebSocketUrl(url: string): boolean {
+  let parsed: URL;
+  try {
+    parsed = new URL(url);
+  } catch {
+    return false;
+  }
+
+  if (parsed.protocol === "wss:") {
+    return true;
+  }
+
+  if (parsed.protocol !== "ws:") {
+    return false;
+  }
+
+  // ws:// is only secure for loopback addresses
+  return isLoopbackHost(parsed.hostname);
+}