tests: add edge-case tests and record test runtime:API test and Mocked API test

[VadimLindebaum]

[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 (11349ms)
Mocked tests (app.mock.test.js): ✅ PASS (1232ms)
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/24340475635
PR checks: https://github.com/tanjaq/User-Register/pull/37/checks