Update backend unit tests coverage for kernelci_cache

Currently, the backend unit test coverage is around 40%.
The goal is to increase this coverage by adding tests across the entire backend codebase.

## General
- [ ] **checkouts.py** - current: 0%
- [ ] **utils.py** - current: 33%

## Queries
- [ ] **issues.py** - current: 20%
- [ ] **notifications.py** - current: 27%
- [ ] **tree.py** - current: 38%

# Related Issues
- #1494

Start by reviewing the backend unit-test layout and the listed files: checkouts.py, utils.py, issues.py, notifications.py, and tree.py. Run the existing coverage workflow to confirm the reported baseline, then add tests for uncovered behavior across those files. Done means the backend coverage is increased and the new tests pass.

Written by the indexing model from the issue text.