fix: include rollout_path in NewConversationResponse (#3352)

Adding the `rollout_path` to the `NewConversationResponse` makes it so a
client can perform subsequent operations on a `(ConversationId,
PathBuf)` pair. #3353 will introduce support for `ArchiveConversation`.

---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/3352).
* #3353
* __->__ #3352

codex-rs/protocol/src/mcp_protocol.rs

@@ -203,6 +203,7 @@ pub struct NewConversationParams {
 pub struct NewConversationResponse {
     pub conversation_id: ConversationId,
     pub model: String,
+    pub rollout_path: PathBuf,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, TS)]

codex-rs/protocol/src/protocol.rs

@@ -958,6 +958,8 @@ pub struct SessionConfiguredEvent {
     /// When present, UIs can use these to seed the history.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub initial_messages: Option<Vec<EventMsg>>,
+
+    pub rollout_path: PathBuf,
 }
 
 /// User's decision in response to an ExecApprovalRequest.
@@ -1020,12 +1022,15 @@ pub enum TurnAbortReason {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use serde_json::json;
+    use tempfile::NamedTempFile;
 
     /// Serialize Event to verify that its JSON representation has the expected
     /// amount of nesting.
     #[test]
     fn serialize_event() {
         let conversation_id = ConversationId(uuid::uuid!("67e55044-10b1-426f-9247-bb680e5fe0c8"));
+        let rollout_file = NamedTempFile::new().unwrap();
         let event = Event {
             id: "1234".to_string(),
             msg: EventMsg::SessionConfigured(SessionConfiguredEvent {
@@ -1034,13 +1039,22 @@ mod tests {
                 history_log_id: 0,
                 history_entry_count: 0,
                 initial_messages: None,
+                rollout_path: rollout_file.path().to_path_buf(),
             }),
         };
-        let serialized = serde_json::to_string(&event).unwrap();
-        assert_eq!(
-            serialized,
-            r#"{"id":"1234","msg":{"type":"session_configured","session_id":"67e55044-10b1-426f-9247-bb680e5fe0c8","model":"codex-mini-latest","history_log_id":0,"history_entry_count":0}}"#
-        );
+
+        let expected = json!({
+            "id": "1234",
+            "msg": {
+                "type": "session_configured",
+                "session_id": "67e55044-10b1-426f-9247-bb680e5fe0c8",
+                "model": "codex-mini-latest",
+                "history_log_id": 0,
+                "history_entry_count": 0,
+                "rollout_path": format!("{}", rollout_file.path().display()),
+            }
+        });
+        assert_eq!(expected, serde_json::to_value(&event).unwrap());
     }
 
     #[test]