fix: stop logging an error when a chat bot is simply not configured

bot.Run() attempts to configure every registered bot (slack, hipchat)
and logged 'can not get configuration for bot [slack]' at Error level
whenever the Slack bot tokens were absent - the normal state for anyone
not using the Slack bot (e.g. Mattermost-only setups).

- bot.Run() now logs the unconfigured case at Info level with clearer
  wording.
- The Slack bot skips silently (Debug) when no tokens are provided at
  all, and warns only when tokens are present but malformed.

Fixes keel-hq/keel#848

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Spec-Ref: helix-specs@97ae81a6:000345_can-we-look-into