feat: Enterprise Connect implementation

[Piyush-85]

Description

Adds Enterprise Connect (EC) support to the auth0-express SDK.

In EC mode Auth0 acts as a pure SSO relay, it writes no Auth0 session, and the application owns its own session (established in the onCallback hook). This PR adds the full EC surface:

• enterpriseConnect: true config flag, with a required onCallback hook (validated at config time).
• startEnterpriseLogin(req, res, { email, returnTo? }) - email-domain Home Realm Discovery (WebFinger) followed by an interactive login redirect; returns true when federated, false otherwise.
• Re-exports isFederatedDomain and EnterpriseConnectNotSupportedError from @auth0/auth0-server-js, and the EnterpriseConnectCallbackHook type, so integrators can compose the flow themselves (isFederatedDomain + /auth/login?login_hint=).
• EC-aware callback: invokes onCallback with { idTokenClaims, user, appState } and writes no Auth0 session.
• Always-federated logout in EC mode (ends the upstream enterprise IdP session).
• requiresAuth() is documented as unsupported in EC mode (session-backed methods throw EnterpriseConnectNotSupportedError; enforcement lives in auth0-server-js).

The public API and usage are documented in the SDK docs - README (Features table) and EXAMPLES.md → Enterprise Connect — not repeated here.

References

• Runnable example (separate PR, targets this branch): examples/example-express-enterprise-connect

Testing

• Added unit/integration coverage for config validation, the EC callback path (onCallback invoked, no session cookie, appState forwarded, error handling), startEnterpriseLogin (federated / non-federated / returnTo persistence), federated logout, and method enforcement in EC mode.
• Full package suite green: 292 tests passing; lint and build clean.

Environment: Node.js / TypeScript, Express 5, macOS.

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs — README + EXAMPLES included
• All active GitHub checks for tests, formatting, and security are passing