Unify realtime v1/v2 session config (#14606)

## Summary
- unify realtime websocket settings under `[realtime]` (`version` and
`type`)
- remove `realtime_conversation_v2` and select parser/session mode from
config

## Testing
- not run (per request)

---------

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

codex-rs/core/config.schema.json

@@ -426,9 +426,6 @@
             "realtime_conversation": {
               "type": "boolean"
             },
-            "realtime_conversation_v2": {
-              "type": "boolean"
-            },
             "remote_models": {
               "type": "boolean"
             },
@@ -1342,6 +1339,18 @@
       },
       "type": "object"
     },
+    "RealtimeToml": {
+      "additionalProperties": false,
+      "properties": {
+        "type": {
+          "$ref": "#/definitions/RealtimeWsMode"
+        },
+        "version": {
+          "$ref": "#/definitions/RealtimeWsVersion"
+        }
+      },
+      "type": "object"
+    },
     "RealtimeWsMode": {
       "enum": [
         "conversational",
@@ -1349,6 +1358,13 @@
       ],
       "type": "string"
     },
+    "RealtimeWsVersion": {
+      "enum": [
+        "v1",
+        "v2"
+      ],
+      "type": "string"
+    },
     "ReasoningEffort": {
       "description": "See https://platform.openai.com/docs/guides/reasoning?api-mode=responses#get-started-with-reasoning",
       "enum": [
@@ -1827,14 +1843,6 @@
       "description": "Experimental / do not use. Overrides only the realtime conversation websocket transport base URL (the `Op::RealtimeConversation` `/v1/realtime` connection) without changing normal provider HTTP requests.",
       "type": "string"
     },
-    "experimental_realtime_ws_mode": {
-      "allOf": [
-        {
-          "$ref": "#/definitions/RealtimeWsMode"
-        }
-      ],
-      "description": "Experimental / do not use. Selects the realtime websocket intent mode. `conversational` is speech-to-speech while `transcription` is transcript-only."
-    },
     "experimental_realtime_ws_model": {
       "description": "Experimental / do not use. Selects the realtime websocket model/snapshot used for the `Op::RealtimeConversation` connection.",
       "type": "string"
@@ -1959,9 +1967,6 @@
         "realtime_conversation": {
           "type": "boolean"
         },
-        "realtime_conversation_v2": {
-          "type": "boolean"
-        },
         "remote_models": {
           "type": "boolean"
         },
@@ -2309,6 +2314,15 @@
       },
       "type": "object"
     },
+    "realtime": {
+      "allOf": [
+        {
+          "$ref": "#/definitions/RealtimeToml"
+        }
+      ],
+      "default": null,
+      "description": "Experimental / do not use. Realtime websocket session selection. `version` controls v1/v2 and `type` controls conversational/transcription."
+    },
     "review_model": {
       "description": "Review model override used by the `/review` feature.",
       "type": "string"