Add realtime output modality and transcript events (#17701)

- Add outputModality to thread/realtime/start and wire text/audio output
selection through app-server, core, API, and TUI.\n- Rename the realtime
transcript delta notification and add a separate transcript done
notification that forwards final text from item done without correlating
it with deltas.

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -1525,6 +1525,13 @@
         }
       ]
     },
+    "RealtimeOutputModality": {
+      "enum": [
+        "text",
+        "audio"
+      ],
+      "type": "string"
+    },
     "RealtimeVoice": {
       "enum": [
         "alloy",