[App Server] Allow fetching or resuming a conversation summary from the conversation id (#5890)

This PR adds an option to app server to allow conversation summaries to
be fetched from just the conversation id rather than rollout path for
convenience at the cost of some latency to discover the rollout path.

This convenience is non-trivial as it allows app servers to simply
maintain conversation ids rather than rollout paths and the associated
platform (Windows) handling associated with storing and encoding them
correctly.

codex-rs/app-server/tests/suite/list_resume.rs

@@ -171,7 +171,8 @@ async fn test_list_and_resume_conversations() -> Result<()> {
     // Now resume one of the sessions and expect a SessionConfigured notification and response.
     let resume_req_id = mcp
         .send_resume_conversation_request(ResumeConversationParams {
-            path: items[0].path.clone(),
+            path: Some(items[0].path.clone()),
+            conversation_id: None,
             overrides: Some(NewConversationParams {
                 model: Some("o3".to_string()),
                 ..Default::default()