Two related Slack delivery fixes for send_message text sends:
- Route Slack text delivery through _send_via_adapter so the live
in-process gateway adapter (multi-workspace aware, channel→client
mapping, adapter-side gates) is preferred, with the plugin's
_standalone_send as the out-of-process fallback — matching how the
media path already behaves.
- _standalone_send: SLACK_BOT_TOKEN can be a comma-separated list in
multi-workspace installs and slack_tokens.json carries OAuth
per-workspace tokens; the standalone Web-API path used to send the
literal comma-joined string, which Slack rejects as invalid_auth.
Try each token individually, retrying on token-scoped errors
(invalid_auth / not_in_channel / channel_not_found …) and stopping on
terminal ones. User-DM resolution (U…/W… targets) also tries each
token.
Adapted from #47547 by @replygirl — the original patched the legacy
tools/send_message_tool.py::_send_slack helper, which moved to the
Slack plugin's _standalone_send in #41112.
Salvaged from #47547
