Create integration tests for the external app

**Is your feature request related to a problem? Please describe.**
There is not enough testing for the cornflow-base to external-app connection.

**Describe the solution you'd like**
Develop integration tests for this case.

Start by locating the cornflow-base to external-app connection entry points and reviewing the existing test layout. Identify the integration boundary and any current connection-related tests, then define coverage for the external-app case. Done means the integration tests exercise that connection and pass reliably.

Written by the indexing model from the issue text.