fix: missing Slack notifications for group request created and completed events

[elaynelemos]

Adds the two missing hook implementations to Slack notification plugin (examples/plugins/notifications_slack/notifications.py):

• access_group_request_created: DMs each approver, DMs the requester when they aren't already in the approver list, and posts to the alerts channel. Links to /group-requests/{id}.
• access_group_request_completed: DMs the requester when notify_requester=True, DMs each approver other than the requester, and posts to the alerts channel. Uses the created group's name when approved, falling back to the resolved/requested name on rejection.