setDraftMessage

Updates the in-memory draft immediately. The durable SavedStateHandle write is debounced by DRAFT_PERSISTENCE_DELAY_MS — rapid edits cancel the prior pending flush, so only the trailing value persists. On process death within the debounce window the last ≤DRAFT_PERSISTENCE_DELAY_MS of typing is not durably saved; this is a conscious trade-off to avoid per-keystroke jank.