Refactor: Added Service Layer for EmployeeController

[supalvasani]

Pull Request Template

Description

Refactored the codebase by extracting business logic from EmployeeController and moving it into a new EmployeeService class. This improves code readability, modularity, and makes the controller more concise and maintainable.

Also updated the controller methods to use service layer methods.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Refactor (non-breaking change that improves code structure)

✅ How Has This Been Tested?

• Verified all existing endpoints (GET, POST, PUT, DELETE) using Postman.
• Ensured that API responses remain unchanged after refactoring.
• Keploy test generation and replay will be done after this PR is reviewed/merged.

🧩 New Additions

• Integrated Swagger UI for better API documentation and testing.
  • Available at: http://localhost:8080/swagger-ui/index.html

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings
• I have tagged the reviewers in a comment below incase my pull request is ready for a review
• I have signed the commit message to agree to Developer Certificate of Origin (DCO) (to certify that you wrote or otherwise have the right to submit your contribution to the project.) by adding "--signoff" to my git commit command.

[supalvasani]

Added a service layer to extract business logic from the controller.
Improves maintainability and separation of concerns.
Will add Keploy tests after resolving Java version issues.
Ready for review.

[supalvasani]

I've fixed the DCO issue and force-pushed the signed commit.
Please review and approve the workflow when convenient. 🙏

[pratik-mahalle]

Hey @supalvasani, Thank you for raising the pr. Our team will review this pr soon