feat: #3584548 add deferred event sending via kernel.terminate

Frank Mablyrequested to merge
issue/pianoanalytics-3584548:3584548-add-deferred-event into 2.x

Add queueEvent() to ServerSideEventSender for non-blocking server-side tracking. Queued events are sent after the response via DeferredEventSubscriber on kernel.terminate.

sendEvent() remains available for synchronous use cases. Integration modules call queueEvent() instead of managing their own terminate subscribers.

Closes #3584548

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