[mcp-server] Add reply tool call (#1643)

## Summary
Adds a new mcp tool call, `codex-reply`, so we can continue existing
sessions. This is a first draft and does not yet support sessions from
previous processes.

## Testing
- [x] tested with mcp client

codex-rs/core/src/codex.rs

@@ -101,7 +101,7 @@ impl Codex {
     /// Spawn a new [`Codex`] and initialize the session. Returns the instance
     /// of `Codex` and the ID of the `SessionInitialized` event that was
     /// submitted to start the session.
-    pub async fn spawn(config: Config, ctrl_c: Arc<Notify>) -> CodexResult<(Codex, String)> {
+    pub async fn spawn(config: Config, ctrl_c: Arc<Notify>) -> CodexResult<(Codex, String, Uuid)> {
         // experimental resume path (undocumented)
         let resume_path = config.experimental_resume.clone();
         info!("resume_path: {resume_path:?}");
@@ -124,7 +124,12 @@ impl Codex {
         };
 
         let config = Arc::new(config);
-        tokio::spawn(submission_loop(config, rx_sub, tx_event, ctrl_c));
+
+        // Generate a unique ID for the lifetime of this Codex session.
+        let session_id = Uuid::new_v4();
+        tokio::spawn(submission_loop(
+            session_id, config, rx_sub, tx_event, ctrl_c,
+        ));
         let codex = Codex {
             next_id: AtomicU64::new(0),
             tx_sub,
@@ -132,7 +137,7 @@ impl Codex {
         };
         let init_id = codex.submit(configure_session).await?;
 
-        Ok((codex, init_id))
+        Ok((codex, init_id, session_id))
     }
 
     /// Submit the `op` wrapped in a `Submission` with a unique ID.
@@ -521,14 +526,12 @@ impl AgentTask {
 }
 
 async fn submission_loop(
+    mut session_id: Uuid,
     config: Arc<Config>,
     rx_sub: Receiver<Submission>,
     tx_event: Sender<Event>,
     ctrl_c: Arc<Notify>,
 ) {
-    // Generate a unique ID for the lifetime of this Codex session.
-    let mut session_id = Uuid::new_v4();
-
     let mut sess: Option<Arc<Session>> = None;
     // shorthand - send an event when there is no active session
     let send_no_session_event = |sub_id: String| async {