detectMention fails in multi-workspace OAuth mode, causing onNewMention to never fire when message.channels is subscribed

[mlenczewski-tidio]

Bug Description

When using multi-workspace OAuth (no defaultBotToken), detectMention() always returns false because adapter.botUserId is undefined. This is harmless when only app_mention events are subscribed (the Slack adapter sets isMention = true directly for app_mention events). However, when message.channels is also subscribed, the message event arrives before app_mention, wins the dedupe check, and routes to onNewMessage instead of onNewMention. The subsequent app_mention event is then silently dropped by deduplication.

The root cause is in detectMention() — it relies on adapter.botUserId which is only populated when defaultBotToken is configured (via auth.test in initialize()). In multi-workspace OAuth mode where tokens are per-workspace, _botUserId is never set from auth.test, and the per-context botUserId is not available during handleIncomingMessage.

Steps to Reproduce

1. Create a Chat instance with multi-workspace OAuth (no defaultBotToken)
2. Subscribe to both message.channels and app_mention in Slack Event Subscriptions
3. Register both onNewMention and onNewMessage(/.*/) handlers
4. @mention the bot in a public channel
5. Observe that onNewMention handler is never called; onNewMessage handler fires instead

Expected Behavior

When a user @mentions the bot in a public channel, onNewMention should fire. The message event should be routed to onNewMention because the bot's user ID is present in the message text (<@BOT_USER_ID>).
The app_mention event being deduped is acceptable as long as the message event is correctly routed.

Actual Behavior

detectMention() returns false because adapter.botUserId is undefined in multi-workspace OAuth mode. The message event falls through to onNewMessage pattern matching (step 4 in the routing chain). The
app_mention event is then dropped by deduplication (same ts). The onNewMention handler never fires.

Code Sample

import {Chat} from 'chat';
import {createSlackAdapter} from '@chat-adapter/slack';
import {createRedisState} from '@chat-adapter/state-redis';

// Multi-workspace OAuth — no defaultBotToken
const bot = new Chat({
    userName: 'mybot',
    adapters: {
        slack: createSlackAdapter({
            signingSecret: process.env.SLACK_SIGNING_SECRET!,
            clientId: process.env.SLACK_CLIENT_ID!,
            clientSecret: process.env.SLACK_CLIENT_SECRET!,
            encryptionKey: process.env.SLACK_ENCRYPTION_KEY!,
        }),
    },
    state: createRedisState({url: process.env.REDIS_URL!}),
});

// This handler NEVER fires when message.channels is subscribed
bot.onNewMention(async (thread, message) => {
    console.log('Mention received!'); // Never logged
    await thread.post('Hello!');
});

// This handler catches everything, including mentions
bot.onNewMessage(/.*/, async (thread, message) => {
    console.log('New message:', message.text); // Fires for mentions too
});

Chat SDK Version

4.23.0

Node.js Version

No response

Platform Adapter

Slack

Operating System

None

Additional Context

No response

[mdnanocom]

This PR fixes it #328