Add per-session realtime model and version overrides (#24999)

## Why

Clients need to select a realtime session configuration for an
individual start without rewriting persisted configuration or restarting
the app-server process.

## What Changed

- Add optional `model` and `version` fields to `thread/realtime/start`
- Forward those optional values through the realtime start operation and
apply them only for that session
- Preserve existing configured/default behavior when the new fields are
omitted
- Update generated protocol schema and app-server documentation

## Validation

- Added/updated protocol serialization coverage for the new optional
request fields
- Added focused core coverage for a session override taking precedence
over configured realtime selection
- Added focused app-server coverage that a request override reaches the
realtime WebSocket handshake

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

@@ -1993,6 +1993,13 @@
       ],
       "type": "object"
     },
+    "RealtimeConversationVersion": {
+      "enum": [
+        "v1",
+        "v2"
+      ],
+      "type": "string"
+    },
     "RealtimeOutputModality": {
       "enum": [
         "text",