Add Jest testing infrastructure to demo JavaScript application

[Copilot]

This PR adds comprehensive testing infrastructure to the demo JavaScript application in response to the "testdemo" issue.

Changes

Testing Framework

• Added Jest as the testing framework with supertest for HTTP endpoint testing
• Installed as dev dependencies to keep the production bundle clean

Code Modifications

• Modified server.js to export the Express app for testing while maintaining backward compatibility
• Wrapped app.listen() in a conditional check (if (require.main === module)) so the server only starts when run directly, not when imported for testing

Test Coverage

Created server.test.js with a basic test suite that validates:

• The root endpoint (/) returns HTTP 200 status
• The response body contains the expected "Hello World!" message

Developer Experience

• Added npm test script to package.json for easy test execution
• Updated README.md with a "Running Tests" section providing clear instructions

Usage

cd app
npm install
npm test

Verification

✅ All tests pass successfully
✅ Server still starts and runs correctly with npm start
✅ No existing functionality was broken or modified
✅ Application remains fully compatible with Docker and Kubernetes deployment

This provides a foundation for future test expansion while maintaining the simplicity and functionality of the demo application.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>testdemo</issue_title>
<issue_description>test</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #1

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