test: Add 72 unit tests for auth, patient, and medicine services

• Clear Description: This merge request adds comprehensive unit tests for critical service-layer logic in the eHRS FastAPI backend. The goal is to validate authentication, patient management, and medicine inventory services using isolated, mock-based unit tests, ensuring correctness without relying on a real database. The tests focus on:

• Authentication workflows (password hashing, JWT tokens, OTP-based reset)
• Patient lifecycle operations (CRUD, registration, vitals, search)
• Medicine inventory management (creation, updates, stock validation)

These changes improve code reliability, test coverage, and regression safety, especially for core business logic that underpins the API layer.

• Checklist: A checklist of completed tasks, such as:
  • Code follows project API guidelines (if applicable).
  • Tests are included and passing.
  • Documentation is updated (if applicable).
  • Code adheres to project coding standards.
• Related Issue(s): Link to any relevant issues using Closes #19 .