new(endpoints): add authenticator first login flow

[julia-rabello]

Summary

• Adds the three endpoints that let a newly created storefront user complete their first login: POST Start login attempt, POST Sign in, and POST Set password.
• These were missing from the Authenticator API split (feat: split Authenticator API out of VTEX ID API #1752); Create User already existed but had no way to document how the user actually sets their first password.

Test plan

• JSON validated
• Spectral lint (.spectral.yml) passes with 0 errors/warnings

[vtex-pr-sentinel]

🛡️ SDD Check — action required

I couldn't detect an SDD in this PR. Please check one option below (requires write access to the repo):

• SDD lives in another PR — paste the SDD PR URL here:
• This PR doesn't need an SDD
• SDD applies, but I'm not adopting it in this PR

[github-actions]

OpenAPI Preview

Preview the changed specs:

• VTEX - Authenticator API.json

Links point to commit 950b7e1. They will update on each new push to this PR.

[github-actions]

Thanks for your contribution. The .json file will be checked now with Spectral.

[ricardoaerobr]

LGTM!

[julia-rabello]

Decidimos não divulgar publicamente todas as rotas do fluxo de first login B2B por enquanto, apenas as rotas de criação de storefront user e de get serão documentadas (a rota de get via doc request separada). Por isso, fechando este PR. O fluxo completo foi documentado e salvo em PDF internamente, sem publicação, para evitar confusão. Podemos reabrir se a decisão mudar.