feat(react): create user mfa hook

[NaveenChand755]

Summary

Added useUserMFA — a public hook that owns the full MFA management lifecycle: enrollment phase state, dialog state, and centralised error handling with MFA step-up awareness across fetch, enroll, confirm, and delete.

Why

Existing useMFA/useMFALogic hooks are coupled to component internals — not usable as a public API
Error handling was scattered with no MFA step-up awareness on enroll/confirm paths

What

New useUserMFA hook exposing enrollment phases, dialog state, and all MFA operations
All errors route through one place — step-up/5xx silently to GateKeeper, factor-specific codes toasted with i18n resolver

Packages

• packages/core
• packages/react
• examples

References

Testing

How can this be verified? Note anything intentionally not covered by tests and why.

• This change adds unit test coverage
• Tested for both SPA and RWA flows, all example apps working
• All existing and new tests complete without errors

Checklist

• Breaking change
• Requires docs update
• Backward compatible

Contributing

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct