feat: support sending login, logout, identify, and modify_user events

[khushi1033]

Summary

Implements onLoginComplete, onLogoutComplete, onUserIdentified, and onModifyComplete kit callbacks so all four identity lifecycle events are forwarded to the Rokt v2/sessions events pipeline as user_login, user_logout, user_identified, and user_modified respectively.

Each callback updates self.userAttributes from the incoming user before firing — ensuring _enrichEvent carries post-identity-change attributes — then calls _sendEventStream, which queues the event if the launcher isn't ready yet. A shared _buildIdentityEvent helper constructs the event payload with MPID, SessionId, UserAttributes, and UserIdentities for all four callbacks.

Note: user_login, user_logout, user_identified, and user_modified are not yet in the v2 event registry; the API accepts them with a warning.

Testing Plan

Unit tests: added for all four callbacks covering: attribute update, successful stream forwarding with correct payload (including MPID, SessionId, UserAttributes, UserIdentities), queuing when event_stream is unavailable, and queuing when window.Rokt is undefined. Edge cases for null MPID and null SessionId are also covered on the login callback (shared behaviour via _buildIdentityEvent).

Manual: mParticle.Identity.login() / logout() on local sample app and confirmed POST /v2/sessions/events requests appear with the correct eventType in the network tab. mParticle.Identity.identify() and mParticle.Identity.modify() were called via the browser console to verify user_identified and user_modified events respectively.

[jamesnrokt]

We need to confirm these match what's expected