Add notify to code-mode (#14842)

Allows model to send an out-of-band notification.

The notification is injected as another tool call output for the same
call_id.

codex-rs/app-server-protocol/schema/json/v2/ThreadResumeParams.json

@@ -673,6 +673,12 @@
             "call_id": {
               "type": "string"
             },
+            "name": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
             "output": {
               "$ref": "#/definitions/FunctionCallOutputBody"
             },