fix(notifications): use regression event data instead of stale group metadata

[sentry-junior]

Problem

When an issue regresses, both email and Slack notifications show the original issue's error message instead of the event that actually triggered the regression.

For example:

• Issue originally triggered by ValueError: missing field 'name'
• Issue gets resolved
• New event ValueError: timeout connecting to database triggers regression
• Notification shows ValueError: missing field 'name' ← wrong

This was reported by a customer: "the original message from an issue gets posted over and over again even though the latest issue in an issue group is a totally different one."

Root Cause

Two compounding issues:

1. RegressionActivityNotification ignores the regression event — the Activity.data stores event_id but the notification code never reads it. All downstream renderers fall back to group-level metadata.
2. Notification fires before group metadata is updated — _handle_regression triggers the async notification, then _process_existing_aggregate updates metadata via buffer_incr (also async). The notification task reads stale group data.

Fix

• Load the regression event from activity.data['event_id'] in RegressionActivityNotification.__init__ using eventstore.backend.get_event_by_id()
• Override build_attachment_title and get_title_link to use the regression event when available
• Pass event to email template context via get_group_context override
• Update _group.html to prefer event metadata over group metadata when present (backwards-compatible via |default filter)
• Graceful fallback: if event lookup fails (deleted, unavailable), falls back to existing group-based behavior

Changes

File	Change
src/sentry/notifications/notifications/activity/regression.py	Load regression event, override title/link/context methods
src/sentry/templates/sentry/emails/_group.html	Prefer event metadata when available
tests/.../slack/notifications/test_regression.py	Add tests for regression event metadata usage and fallback

Testing

• test_regression_uses_regression_event_metadata — verifies Slack notification shows regression event's error message
• test_regression_without_event_id_falls_back_to_group — verifies graceful fallback when no event_id present
• All existing regression notification tests remain unchanged

Requested-by: Daniel Griesser daniel@sentry.io