refactor(slack): preserve SDK types in slash command wiring (#144734)

refactor(slack): preserve SDK types in slash command wiring (#144734)

Replace three private forwarding modules with named re-exports while keeping
their lazy module paths. Model the existing optional partial-delivery cause
in its shared envelope and remove Slack's unnecessary Error assertion.

Preserve command registration, reply settlement, and structural/cross-realm
cause handling. Update the SDK documentation and exact assertion baseline.