Slack Workflow Builder posts (and other app/bot messages) arrive as
subtype=bot_message with user=None. _is_user_authorized rejected them at
the `if not user_id: return False` guard, which runs *before* the #4466
{PLATFORM}_ALLOW_BOTS bypass — so @mentioning the bot from a Slack
workflow silently did nothing, even with SLACK_ALLOW_BOTS (or
SLACK_ALLOW_ALL_USERS) set. The chat-scoped allowlist for Telegram/QQ
already runs before that guard for the same reason (channel broadcasts
with no from_user); Slack was both missing from the bot-bypass map and
had the bypass running too late.
- gateway/authz_mixin: move the {PLATFORM}_ALLOW_BOTS bypass ahead of the
no-user-id guard and add Platform.SLACK -> SLACK_ALLOW_BOTS.
- plugins/platforms/slack/adapter: set is_bot=True on inbound
bot_message events so the gateway can identify workflow/app senders
(they carry no user_id to match against the allowlist).
Tested: new tests/gateway/test_slack_bot_auth_bypass.py plus the existing
Discord/Feishu bot-auth and gateway authz/gating suites all pass.
d3c8667462