[Feature] Add pytest with mocked Twilio for contributor-friendly testing

[sindhugattigoppula]

Feature Description

current testing requires Twilio credentials and verified numbers, creating barriers for contributors.
Solution:
Add a robust pytest suite that:
Mocks Twilio API responses 100% locally
Validates all OTP flows (success/failure cases)
Integrates with GitHub Actions for auto-testing

Use Case

They clone the repo.
Run pytest -v --cov=app.
The mocked Twilio client instantly simulates both OTP send and verify calls, including success and failure paths.
They see green tests in under 3 seconds — without needing any credentials or internet. They can confidently open a PR knowing it passes the OTP workflow checks.

Contributors can start contributing in <5 minutes without setup headaches.
Maintainers get PRs that are already tested and reliable.
CI/CD pipelines won’t fail if Twilio services are down.

[sindhugattigoppula]

Please assign this to me @Harish-2003