`nrpe-external-master` endpoint has no requirers

This time no requirers, only providers, and lots of them: https://charmhub.io/integrations/nrpe-external-master

Example run: https://test-observer.canonical.com/#/charms/407679?testExecutionId=512349&testResultId=11071789

Start by reproducing the linked Test Observer execution and reviewing the nrpe-external-master integration coverage in this pytest-based repository. Compare the endpoint's provider-only behavior with the test's expected requirers, then identify the relevant test change and verify the example scenario passes.

Written by the indexing model from the issue text.