[SUBTASK]-Enhance GroupController Implementation

[cagataycode]

Description

Group controller is missing some key endpoints and functionality.

Tasks

1. Add /leave endpoint to allow users to leave a group.
2. Add /members endpoint to retrieve a list of group members.
3. Update /group/join to retrieve the invite code.
4. Add /group-info endpoint to retrieve group details, including saved join code and related group data.

Acceptance Criteria

• Implement a /leave endpoint in Group controller
• Ensure the /members endpoint retrieves the correct list of group members
• Ensure /group/join retrieves the invite code upon request.
• Implement /group-info to return group details, including the saved join code.