Add unit tests for the application

[thomasiverson]

Summary

Add unit tests to improve code quality and reliability for the GitHub API Explorer application.

Tasks

• Set up a testing framework (e.g., Jest / Vitest) with Next.js 16 and React 19 compatibility
• Add unit tests for API route handlers (execute, graphql, environments, history, collections, etc.)
• Add unit tests for library modules (auth, db, types, templates, webhooks)
• Add unit tests for React components (Sidebar, RequestBuilder, ResponseViewer, TopBar, etc.)
• Add a test script to \package.json`n- [ ] Configure CI to run tests on PRs

Acceptance Criteria

• All API routes have basic request/response tests
• Core library functions have unit test coverage
• Components render without errors
• Tests pass in CI