refactor: update cursor request handling and improve parent request management

- Changed `run_id` to `request_id` in `CursorParent` struct for clarity.
- Enhanced the `prepare` function to handle parent requests asynchronously, ensuring proper error handling for active runs.
- Updated database interactions to include `cursor_request_id` for better tracking of requests.
- Added tests to verify the behavior of reused cursor request IDs and their mapping to distinct executions.

server/tests/checkpoint_recovery.rs

@@ -169,8 +169,16 @@ async fn eligible_pending_checkpoint_resumes_tools_before_the_next_model_call()
         1,
         "resume must execute the pending batch before calling the model"
     );
+    let resumed_run_id = store
+        .active_run_for_cursor_request("resumed-run")
+        .await
+        .unwrap()
+        .unwrap();
     let resumed_round = store
-        .tool_round(&ToolRoundId::new("resumed-run:round:resume"))
+        .tool_round(&ToolRoundId::new(format!(
+            "{}:round:resume",
+            resumed_run_id.as_str()
+        )))
         .await
         .unwrap()
         .unwrap();