While porting https://github.com/osbuild/images/pull/1907 to osbuild-composer, I ran into the problem that the CloudAPI endpoint for compose manifests was not checking manifest generation job results and would return a 'nil' value instead of an error if the manifest generation failed.

Moreover, the mock data used by unit tests for depsolved packages was not complete and was lacking basic properties, such as the remote location URL, which is needed to generate an osbuild manifest.

This PR addresses the issues mentioned above, it also adds a unit test for compose manifests Cloud API endpoint and improves unit tests where failure debugging was previously unnecessarily complicated.

/jira-epic HMS-8910