refactor(vercel): normalize env responses and helpers

Introduce utilities to robustly handle Vercel API response shapes:
- add normalizeTarget to coerce target values into string arrays
- add extractEnvs to safely pull envs arrays from union responses

Replace ad-hoc existence checks and inline normalization with these helpers
in getVercelEnvironmentVariables and getVercelEnvironmentVariableValues to
avoid runtime errors when API returns varying shapes (string vs array vs
missing). This simplifies mapping logic and centralizes normalization.

Remove several outdated JSDoc comments and compress inline documentation to
keep the implementation focused and more readable.

Author: 0ski