Define unit-testing conventions and contributor guidance

### Description

Establish conventions for test naming, location, structure, assertions, mocks, and initial coverage expectations, then document them for contributors.

### Action Items

- [ ] Define test file naming and placement conventions.
- [ ] Document expectations for assertions, fixtures, and mocks.
- [ ] Define initial coverage priorities and boundaries.
- [ ] Publish concise contributor guidance.

### Additional Information

Parent Epic: https://github.com/hackforla/lucky-parking/issues/732

No files, tests, or test framework are named. Start by reviewing the repository's existing test layout and contributor-facing documentation, then compare current practice with the listed conventions; done means concise published guidance covering naming, placement, structure, assertions, fixtures, mocks, and initial coverage boundaries.

Written by the indexing model from the issue text.