The `validateProviderConfig()` function now checks both config values
AND environment variables when validating provider credentials. This
aligns the validation behavior with `resolveProvider()` which already
falls back to env vars.
Previously, users who set credentials via environment variables would
get validation errors even though the credentials would be found at
runtime. The error messages correctly suggested env vars as an
alternative, but the validation didn't actually check them.
Affects all three supported providers: Twilio, Telnyx, and Plivo.
Fixes#1709
Co-Authored-By: Claude <noreply@anthropic.com>
