Use chatterbox for Slack notifications instead of chat.postMessage

Replace the direct Slack Web API calls (chat.postMessage + SLACK_BOT_TOKEN /
channel ID) with the repo's existing chatterbox convention, matching
linter-failure-notifier.yml: POST to ${CHATTERBOX_URL}/topics/%23api-platform
with basic auth (CHATTERBOX_TOKEN). Renames the post-to-slack-channel safe
output to post-to-chatterbox and drops the SLACK_BOT_TOKEN /
API_PLATFORM_SLACK_CHANNEL_ID secrets/vars. Recompiled the lock with gh aw.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: dcc1f083-b349-4398-ae7e-e2b90b1924b7

Author: jencarlucci