Add realtime transport config (#17097)

Adds realtime.transport config with websocket as the default and webrtc
wired through the effective config.

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

codex-rs/core/config.schema.json

@@ -1497,6 +1497,9 @@
     "RealtimeToml": {
       "additionalProperties": false,
       "properties": {
+        "transport": {
+          "$ref": "#/definitions/RealtimeTransport"
+        },
         "type": {
           "$ref": "#/definitions/RealtimeWsMode"
         },
@@ -1506,6 +1509,13 @@
       },
       "type": "object"
     },
+    "RealtimeTransport": {
+      "enum": [
+        "webrtc",
+        "websocket"
+      ],
+      "type": "string"
+    },
     "RealtimeWsMode": {
       "enum": [
         "conversational",