Increase unit testing coverage in the harness package

[damccorm]

Increase unit testing coverage in the harness package

We want code coverage at or above 50%, it is currently at 43%.

There are a number of tests already present for this package, most of this task is finding the gaps in code coverage and writing test cases for them.

Imported from Jira BEAM-13902. Original Jira may contain additional context.
Reported by: jrmccluskey.
Subtask of issue #21383

[zhenglin-charlie-li]

.take-issue

[MansiSingh17]

Hi, I’m Mansi Singh, prospective GSoC 2026 contributor interested in Kafka Streams Runner.
I’d like to work on this issue if no one else is assigned. Thanks!