Align main realtime v2 wire and runtime flow

Port the realtime v2 session, parser, app-server, and conversation runtime behavior onto the split websocket-method base without pulling in the startup-context or TUI playback slices.

Co-authored-by: Codex <noreply@openai.com>

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

@@ -2771,6 +2771,12 @@
         "data": {
           "type": "string"
         },
+        "itemId": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "numChannels": {
           "format": "uint16",
           "minimum": 0.0,