feat(auth0-server-js): add multi-factor authentication (MFA) support

[amitsingh05667]

This multi-factor authentication (MFA) support is split into two PR's as mentioned below -

• auth0-auth-js - feat(auth0-auth-js): add multi-factor authentication (MFA) support #179
• auth0-server-js - feat(auth0-server-js): add multi-factor authentication (MFA) support #164 (the current one)

Summary

• Introduce ServerMfaClient module with comprehensive Multi-Factor Authentication (MFA) support for server-side flows
• Add four public methods: listAuthenticators(), enrollAuthenticator(), challengeAuthenticator(), and verify() (verifies MFA challenge and saves tokens into session)
• Integrate ServerMfaClient as serverClient.mfa property on ServerClient

Design decisions

• New feature only: Does not introduce breaking changes to existing functionality
• Dedicated error classes: Specific error classes for each MFA operation (enrollment, listing, challenge, verification) for granular error handling
• Server-side only: MFA verify() automatically saves tokens into the user's session after successful verification, keeping credentials server-side
• Delegates to auth0-auth-js: listAuthenticators, enrollAuthenticator, and challengeAuthenticator delegate directly to the underlying AuthClient.mfa, keeping transport logic in one place

Test plan

• 28 unit tests covering all 4 operations in server-mfa-client.spec.ts
• 228 total tests passing across 9 test files
• Error path coverage
• npm run build passes
• Tested end-to-end against a real tenant

[subhankarmaiti]

@amitsingh05667 here are some key point you can consider

1. we should Introduce MFA related error classes for better DX
2. also please update examples

I have an additional comment. So I am dismissing my approval.

[nandan-bhat]

@amitsingh05667
Create separate PRs for auth0-auth-js and auth0-server-js.
Release auth0-auth-js changes first, and use the released version in auth0-server-js.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: ff457196-e135-427e-aa81-349db80a065d

📥 Commits

Reviewing files that changed from the base of the PR and between 277ca4b and 3448c5a.

📒 Files selected for processing (8)

• packages/auth0-server-js/MFA.md
• packages/auth0-server-js/src/index.ts
• packages/auth0-server-js/src/mfa/index.ts
• packages/auth0-server-js/src/mfa/server-mfa-client.spec.ts
• packages/auth0-server-js/src/mfa/server-mfa-client.ts
• packages/auth0-server-js/src/mfa/types.ts
• packages/auth0-server-js/src/server-client.ts
• packages/auth0-server-js/src/test-utils/tokens.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/server-js-mfa-support

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