Feat: PR Assignment Validation & Spam Prevention

### Describe the feature

### Open repositories often receive pull requests linked to issues that are not assigned to the contributor. This leads to:

- Duplicate or conflicting work

- Low-quality / drive-by PRs

- Increased maintainer review burden

- PR spam during hackathons or open calls

### Why?
After this PR raised which is not yet assigned will be automatically labelled as `not-assigned`

### Add ScreenShots

N/A

### Record

- [x] I agree to follow this project's Code of Conduct
- [x] I want to work on this issue