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/tool_loop.rs

@@ -965,7 +965,7 @@ async fn provider_tool_use_waits_for_client_result_then_calls_provider_again() {
     .await
     .unwrap();
     let provider_call_index: i64 =
-        sqlx::query_scalar("SELECT provider_call_index FROM runs WHERE run_id = ?")
+        sqlx::query_scalar("SELECT provider_call_index FROM runs WHERE cursor_request_id = ?")
             .bind("tool-request")
             .fetch_one(&database)
             .await