Add unit tests for authentication API routes

## Description

The authentication API routes in `src/app/api/auth/` currently lack unit tests. Adding tests will improve code reliability and make it safer for contributors to make changes. ## Scope

• Add Jest tests for login, register, and session API routes
• Test validation errors (missing fields, invalid email, wrong password)
• Test successful authentication flow
• Test tenant slug validation ## Getting Started
• Tests go in `__tests__/` or alongside the route files as `*.test.ts`
• Run tes