Add user registration API test suites

[anneli4]

Anneli Peep
Lisasin testid, samad testid mock kujul, mockisin email valideerimise
Jooksuajad:

päris API testid: umbes 30.942 s
mocked API testid: umbes 0.824 s

[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 (31123ms)
Mocked tests (app.mock.test.js): ✅ PASS (1100ms)
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/24831183688
PR checks: https://github.com/tanjaq/User-Register/pull/44/checks