Discord: settle completed button interactions cleanly

[huntharo]

Summary

• I fixed Discord pending input and questionnaire buttons to acknowledge the interaction once and replace the original message with a completion state instead of leaving stale approval buttons behind.
• I tracked whether a Discord interaction was already settled so follow-up messages use followUp when needed and we stop trying to send a second ephemeral reply after the button click has already been acknowledged.
• I expanded the Discord controller coverage around pending-input, questionnaire completion, and plan follow-up behavior so the interaction lifecycle stays exercised in tests.

Validation

• pnpm test
• pnpm typecheck