Default realtime startup to v2 model (#17183)

- Default realtime sessions to v2 and gpt-realtime-1.5 when no override
is configured.
- Add Op::RealtimeConversationStart integration coverage and keep
v1-specific tests explicit.

---------

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

codex-rs/protocol/src/protocol.rs

@@ -1649,8 +1649,8 @@ pub struct HookCompletedEvent {
 #[derive(Debug, Clone, Copy, Default, Deserialize, Serialize, PartialEq, Eq, JsonSchema, TS)]
 #[serde(rename_all = "snake_case")]
 pub enum RealtimeConversationVersion {
-    #[default]
     V1,
+    #[default]
     V2,
 }