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

### Current Behavior

<img width="2043" height="722" alt="Image" src="https://github.com/user-attachments/assets/2d58dbb7-7e03-4186-84d4-68512a11d88c" />

### Proposed Enhancement

<img width="1631" height="416" alt="Image" src="https://github.com/user-attachments/assets/04a1dba8-f45e-40e8-bb31-e6116621661d" />

### Benefits

_No response_

### Additional Context

_No response_