make ServerMessage a discriminated union

Author: tjb9dc

fern/apis/api/openapi.json

@@ -52620,6 +52620,9 @@
         "properties": {
           "message": {
             "description": "These are all the messages that can be sent to your server before, after and during the call. Configure the messages you'd like to receive in `assistant.serverMessages`.\n\nThe server where the message is sent is determined by the following precedence order:\n\n1. `tool.server.url` (if configured, and only for \"tool-calls\" message)\n2. `assistant.serverUrl` (if configure)\n3. `phoneNumber.serverUrl` (if configured)\n4. `org.serverUrl` (if configured)",
+            "discriminator": {
+              "propertyName": "type"
+            },
             "oneOf": [
               {
                 "$ref": "#/components/schemas/ServerMessageAssistantRequest",