[slack] [bug] preserve self mentions in multi-workspace installs

[mdnanocom]

Summary

In multi-workspace Slack installs, the adapter was only preserving self mentions with the adapter-level _botUserId. For OAuth-installed workspaces, the active bot identity is request-scoped, so <@BOT_USER_ID> could be rewritten before mention detection ran.

This change makes self-mention preservation use the effective request-scoped botUserId, so plain Slack message events still trigger onNewMention() correctly in multi-workspace installs.

What changed

• use this.botUserId instead of only _botUserId in resolveInlineMentions()
• add a unit regression for request-scoped multi-workspace bot identity
• add a replay regression covering a plain message event with a bot self mention

this.botUserId already falls back to _botUserId when there is no request-scoped context, so single-workspace behavior stays the same.

Note

The core bug affects plain Slack message events that rely on fallback mention detection. app_mention events are already marked as mentions separately.

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