feat: MFA support in passwordless

[rmad17]

This PR adds MFA handling on top of feat/passwordless-support for embedded passwordless OTP flows.

Added

• Handles Auth0 mfa_required responses from passwordless OTP verification.
• Raises MfaRequiredError with an encrypted MFA token instead of exposing the raw mfa_token.
• Preserves MFA context such as requested audience/scope for follow-up MFA verification.
• Adds support for completing first-login MFA into an SDK-managed session via server_client.mfa.verify(..., persist=True).
• Verifies the final MFA id_token before session creation and reuses the shared session persistence path.
• Improves passwordless start/verify error handling for rate limits, retry-after, non-JSON responses, issuer/audience errors, and missing ID tokens.
• Updates passwordless docs/examples for MFA completion.
• Adds regression coverage for passwordless MFA and expanded passwordless edge cases.

Testing

• This change adds unit test coverage
• This change adds integration test coverage
• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors