Crash with `KeyError` on `changes_count` in kinto-slack

[leplatrem]

https://mozilla.sentry.io/issues/7446707898/?query=is%3Aunresolved&referrer=issue-stream

┆Issue is synchronized with this Jira Task

[leplatrem]

[leplatrem]

Same with comment

Traceback (most recent call last):
  File "pyramid/tweens.py", line 41, in excview_tween
    response = handler(request)
  File "kinto/core/events.py", line 168, in tween
    request.registry.notify(event)
  File "pyramid/registry.py", line 103, in notify
    [_ for _ in self.subscribers(events, None)]
  File "/opt/.venv/lib/python3.14/site-packages/zope/interface/registry.py", line 458, in subscribers
    return self.adapters.subscribers(objects, provided)
  File "/opt/.venv/lib/python3.14/site-packages/zope/interface/adapter.py", line 918, in subscribers
    subscription(*objects)
  File "pyramid/config/adapters.py", line 125, in subscriber_wrapper
    return derived_subscriber(*arg)
  File "pyramid/config/adapters.py", line 97, in derived_subscriber
    return subscriber(arg[0])
  File "__init__.py", line 84, in build_notification
    messages += get_messages(storage, _context)
  File "__init__.py", line 60, in get_messages
    "text": hook["template"].format(**context),
KeyError: 'comment'