feat: detect stale Slack sockets and auto-restart (#30153)

* feat: detect stale Slack sockets and auto-restart

Slack Socket Mode connections can silently stop delivering events while
still appearing connected (health checks pass, WebSocket stays open).
This "half-dead socket" problem causes messages to go unanswered.

This commit adds two layers of protection:

1. **Event liveness tracking**: Every inbound Slack event (messages,
   reactions, member joins/leaves, channel events, pins) now calls
   `setStatus({ lastEventAt, lastInboundAt })` to update the channel
   account snapshot with the timestamp of the last received event.

2. **Health monitor stale socket detection**: The channel health monitor
   now checks `lastEventAt` against a configurable threshold (default
   30 minutes). If a channel has been running longer than the threshold
   and hasn't received any events in that window, it is flagged as
   unhealthy and automatically restarted — the same way disconnected
   or crashed channels are already handled.

The restart reason is logged as "stale-socket" for observability, and
the existing cooldown/rate-limit logic (3 restarts/hour max) prevents
restart storms.

* Slack: gate liveness tracking to accepted events

---------

Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

extensions/slack/src/channel.ts

@@ -427,6 +427,8 @@ export const slackPlugin: ChannelPlugin<ResolvedSlackAccount> = {
         abortSignal: ctx.abortSignal,
         mediaMaxMb: account.config.mediaMaxMb,
         slashCommand: account.config.slashCommand,
+        setStatus: ctx.setStatus as (next: Record<string, unknown>) => void,
+        getStatus: ctx.getStatus as () => Record<string, unknown>,
       });
     },
   },