Drop tests/gateway/test_slack_conversational_senders.py (12 parametrized
cases driving _handle_slack_message end-to-end for allow_bots policies and
canvas mentions — the allow_bots matrix is already covered by the existing
bot-filter tests in test_slack.py) and fold the remaining coverage into two
_prefilter_inbound tests: every housekeeping subtype is dropped, and every
conversational subtype (absent, file_share, thread_broadcast, me_message,
document_mention, bot_message under allow_bots=all) still passes. Distinct
ts per event: the prefilter dedups by (team, ts) before the subtype gate.
Adds the contributor email mapping for the second author.
