feat: Add active_time_on_site_ms and partner_identities to DTO

[rmi22186]

Summary

Re-doing #131 because it was never released and has diverged from master.

[cursor]

PR Summary

Low Risk
Backward-compatible optional TypeScript fields and test-only fixture updates; no runtime logic or security-sensitive behavior changes in this diff.

Overview
Extends the events package TypeScript DTOs with two optional fields aligned to the server event schema.

Batch gains partner_identities, a string map for multiple partner identity key/value pairs (alongside the existing singular partner_identity string). CommonEventData gains active_time_on_site_ms, so any event type that extends common event data can carry active time on site in milliseconds.

Spec fixtures in packages/events/tests/index.spec.ts are updated to exercise both fields on representative Batch and event payloads.

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