* fix(slack): use named import for @slack/bolt App class
The default import `import bolt from '@slack/bolt'` followed by
`const { App } = bolt` doesn't work correctly in Bun due to ESM/CJS
interop issues. The default export comes through as a function rather
than the module object.
Switching to a named import `import { App } from '@slack/bolt'`
resolves the issue and allows the Slack provider to start successfully.
* fix(slack): align Bolt mock with named App export
---------
Co-authored-by: Peter Steinberger <steipete@gmail.com>
