Add realtime voice selection (#17176)

- Add realtime voice selection for realtime/start.
- Expose the supported v1/v2 voice lists and cover explicit, configured,
default, and invalid voice paths.

codex-rs/core/config.schema.json

@@ -1505,6 +1505,9 @@
         },
         "version": {
           "$ref": "#/definitions/RealtimeConversationVersion"
+        },
+        "voice": {
+          "$ref": "#/definitions/RealtimeVoice"
         }
       },
       "type": "object"
@@ -1516,6 +1519,30 @@
       ],
       "type": "string"
     },
+    "RealtimeVoice": {
+      "enum": [
+        "alloy",
+        "arbor",
+        "ash",
+        "ballad",
+        "breeze",
+        "cedar",
+        "coral",
+        "cove",
+        "echo",
+        "ember",
+        "juniper",
+        "maple",
+        "marin",
+        "sage",
+        "shimmer",
+        "sol",
+        "spruce",
+        "vale",
+        "verse"
+      ],
+      "type": "string"
+    },
     "RealtimeWsMode": {
       "enum": [
         "conversational",