Rename Mfa -> MultiFactorAuth, Portal -> AdminPortal

Aligns class names with oagen hint-derived names used across all
SDK languages. Updates directories, files, class names, client
accessors, imports, and tests.

BREAKING CHANGE: workos.mfa -> workos.multiFactorAuth
BREAKING CHANGE: workos.portal -> workos.adminPortal

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>