Add unit tests for index_connection.go

[aulorbe]

Problem

We do not have unit tests for the private functions inside index_connection.go, as outlined in this Asana ticket.

Solution

Add 'em!

Note: I've adjusted the Justfile and renamed the integration test components in accordance with the strategy outlined in this PR (unit tests for client.go).

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] Infrastructure change (CI configs, etc)
• [x] Non-code change (docs, etc)
• [ ] None of the above: (explain here)

Test Plan

CI passes.