Wait for stale startup prewarm abort before MCP swap

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

codex-rs/core/src/session/mcp.rs

@@ -350,7 +350,7 @@ impl Session {
             // The prewarm was built with the stale manager and can hold a read
             // lock while resolving tools. Abort it before swapping managers so
             // an MCP refresh cannot block turn startup behind stale work.
-            startup_prewarm.abort();
+            startup_prewarm.abort_and_wait().await;
         }
 
         let mut old_manager = {

codex-rs/core/src/session_startup_prewarm.rs

@@ -47,8 +47,9 @@ impl SessionStartupPrewarmHandle {
         }
     }
 
-    pub(crate) fn abort(self) {
+    pub(crate) async fn abort_and_wait(self) {
         self.task.abort();
+        let _ = self.task.await;
     }
 
     async fn resolve(