The getSocialProviders endpoint was manually pushing "oidc" to the
providers array when OIDC env vars are present, but Better Auth already
includes the OIDC provider in ctx.context.socialProviders when it is
configured. This resulted in ["oidc","oidc"] being returned from
/api/auth/social-providers, causing a duplicate provider registration
that led to a TypeError: Cannot read properties of null (reading 'id')
during the OIDC login callback, preventing all OIDC logins.
Fix: remove the manual providers.push("oidc") block as it is redundant.