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/src/model/run.rs

@@ -48,6 +48,7 @@ pub struct RecoveredToolRound {
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
 pub struct PreparedRun {
     pub run_id: RunId,
+    pub cursor_request_id: Option<String>,
     pub conversation_id: ConversationId,
     pub kind: RunKind,
     pub model: ModelSpec,