Shuttle module | Register Shuttle

# Create Shuttle Process

### User Story

As a admin, I want to be able to create new shuttle for the company. If the shuttle id already exists for company, the system should display an error message indicating that the shuttle id already exists.

### Task

- [x] Implement Data Access Object (DAO) Layer.
- [x] Implement Service Layer.
- [x] Create JUnit Test Cases.
- [x] Ensure that the tests cases are passing.
- [ ] Write clear and concise comments in the code to explain complex logic.

### DAO:

- createShuttle()

### Service:

- registerShuttle()

### Junit Test:
- testShuttleRegisterSuccess()
- testNullInput()

### Estimation:
- 4hrs

Start by reading the createShuttle() DAO and registerShuttle() service entry points, then inspect testShuttleRegisterSuccess() and testNullInput(). Confirm the existing registration behavior and identify the complex logic that needs clear, concise comments. Done means the comments are added and the listed JUnit tests still pass.

Written by the indexing model from the issue text.