Added tests for server

[kklorfff]

I added tests for the server using Jest and Supertest.

What was done:

Added tests for user registration
Tested different validation cases (username, password, email)
Added coverage report
Fixed tests to make them run correctly

Test command:

npx jest --coverage

Test result:

Test Suites: X passed
Tests: X passed
Time: X.XXX s

Coverage:

Statements: XX%
Branches: XX%
Functions: XX%
Lines: XX%

All tests are passing.

[github-actions]

API & Mocked Tests and Coverage

Purpose: Validate README requirements for regular tests, mocked email tests, coverage, and runtime comparison.
Status: ✅ PASS
Regular tests (app.test.js): ✅ PASS (59429ms)
Mocked tests (app.mock.test.js): ✅ PASS (1342ms)
Coverage gate (both runs at 100%): ✅ PASS
Runtime gate (regular ≥ mocked + 1500ms): ✅ PASS
Regular coverage (app.js): lines=100% | statements=100% | functions=100% | branches=100%
Mocked coverage (app.js only): lines=100% | statements=100% | functions=100% | branches=100%
Run details: https://github.com/tanjaq/User-Register/actions/runs/27421478331
PR checks: https://github.com/tanjaq/User-Register/pull/55/checks

[tanjaq]

OK