[AGE-175] Create tests for llm_apps_service

[mmabrouk]

We had multiple bugs in llm_apps_service in the past. The logic for invoking the applications is quite complex. We need to have tests for the different scenarios and errors. We should use monkey patching to mock some of the results and errors. Most important thing is to make sure the error handling works well.

_{From SyncLinear.com | AGE-175}