Include firstSeen in error event webhook payload

## Summary

The `firstSeen` timestamp is currently available on the issue-level webhook payload but is absent from the error event payload. Teams building error webhook pipelines need `firstSeen` at the event level to determine whether an error is new without a secondary issue lookup.

## Current behavior

The issue webhook payload exposes `firstSeen`, but individual error event webhook payloads do not include this field.

## Gap

Consumers of the error event webhook cannot tell at a glance whether this is the first occurrence of the issue. Correlating `firstSeen` requires a separate API call to the issue endpoint, adding latency and complexity to downstream pipelines.

## Recommendation

Add `firstSeen` (and optionally `lastSeen`) from the parent issue to the error event webhook payload, mirroring what is already available on the issue webhook payload.

Action taken on behalf of Michael Chai.

No file or test path is named. Start by comparing the existing issue webhook payload with the error event webhook payload and trace where each is assembled; then add the required firstSeen behavior, decide whether lastSeen is in scope, and verify the event payload without requiring a secondary issue lookup.

Written by the indexing model from the issue text.