[Enhancement✨] Improve pull_request_hold_service_mut accessor to return Option<&T> in BrokerRuntimeInner

### Current Behavior

<img width="2726" height="1329" alt="Image" src="https://github.com/user-attachments/assets/90f1a32c-9879-4055-b0b7-f6bd0ab0b496" />

### Proposed Enhancement

<img width="2304" height="1012" alt="Image" src="https://github.com/user-attachments/assets/3abfea80-6968-4844-ad10-8c3e581527bd" />

### Benefits

_No response_

### Additional Context

_No response_