Import external agent sessions in background (#20284)

Summary:
- Return from external agent import before session history import
finishes
- Run session import work in the background and emit the existing
completion notification when it is done
- Serialize session imports so duplicate requests do not create
duplicate imported threads

Verification:
- cargo test -p codex-app-server external_agent_config_
- cargo test -p codex-external-agent-sessions
- just fix -p codex-app-server
- just fix -p codex-external-agent-sessions
- git diff --check

codex-rs/app-server/src/codex_message_processor.rs

@@ -518,6 +518,7 @@ impl Drop for ActiveLogin {
 }
 
 /// Handles JSON-RPC messages for Codex threads (and legacy conversation APIs).
+#[derive(Clone)]
 pub(crate) struct CodexMessageProcessor {
     auth_manager: Arc<AuthManager>,
     thread_manager: Arc<ThreadManager>,