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`.

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[//]: # (BEGIN SAPLING FOOTER)
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* #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)]