Dev Task: Improve test coverage of wps_shared package

**Describe the task**
When `wps_shared` was separated out we lost a lot of code coverage. Test in the `api` and `wps_jobs` packages exercise code paths in `wps_shared` but this 'doesn't count' in terms of code coverage when running `wps_shared` tests. We've had to lower the code cov target to 70%.

**Acceptance Criteria**
- [ ] Write tests to get `wps_shared` code cov to > 80%
- [ ] Increase the target threshold in code_cov.yml back to 80%.

**Additional context**
- Add any other context about the task here.
- Or here

Start by running the wps_shared tests with coverage and inspect code_cov.yml to see how the current 70% threshold is configured. Review coverage gaps in wps_shared, using related paths exercised by the api and wps_jobs packages as context. Done means adding tests that raise wps_shared coverage above 80% and restoring the code coverage target to 80%.

Written by the indexing model from the issue text.