fix(welcome-agent): require at least one integration to complete onboarding #993

Description

Follow-up from #885. The engagement criteria in complete_onboarding still allows users to finish onboarding with just 3 exchanges and zero integrations connected. The intent of #885 was to require at least one Composio integration before the welcome agent can complete.

Current behavior

The engagement check uses OR logic — a user can complete onboarding after 3 back-and-forth messages without connecting anything.

Expected behavior

At least 1 Composio integration must be connected before complete_onboarding succeeds. The error message should also be updated to reflect the new requirement.