fix: mark IP as oneOf

IPv4 and IPv6 addresses are mutually exclusive. Marking with `oneOf` fixes
issues with deserializing responses that contain IPs in certain libraries
(ex. progenitor).

Author: abus-sh

main.tsp

@@ -1,5 +1,6 @@
 import "@typespec/http";
 import "@typespec/openapi";
+import "@typespec/openapi3";
 import "@typespec/json-schema";
 import "./variables.tsp";
 
@@ -458,7 +459,11 @@ scalar IPv4 extends string;
 @format("ipv6")
 scalar IPv6 extends string;
 
-alias IP = IPv4 | IPv6;
+@oneOf
+union IP {
+  ipv4: IPv4,
+  ipv6: IPv6,
+}
 
 scalar IPSlashPort extends string;