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/v2/ThreadRealtimeTranscriptDeltaNotification.json

@@ -0,0 +1,23 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "EXPERIMENTAL - flat transcript delta emitted whenever realtime transcript text changes.",
+  "properties": {
+    "delta": {
+      "description": "Live transcript delta from the realtime event.",
+      "type": "string"
+    },
+    "role": {
+      "type": "string"
+    },
+    "threadId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "delta",
+    "role",
+    "threadId"
+  ],
+  "title": "ThreadRealtimeTranscriptDeltaNotification",
+  "type": "object"
+}