test: add more tests to expand coverage

[RyanJWard]

Description

Expand coverage for product_instance.py and conversions.py

Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate unit tests.
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved to the PR if any.
• I have assigned this PR to myself.
• I have added the minimum version decorator to any new backend method implemented.
• I have made sure that the title of my PR follows Conventional commits style (e.g. feat: extrude circle to cylinder)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.66%. Comparing base (8572519) to head (c0f7e94).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2087      +/-   ##
==========================================
+ Coverage   92.85%   93.66%   +0.81%     
==========================================
  Files         138      138              
  Lines        9990     9991       +1     
==========================================
+ Hits         9276     9358      +82     
+ Misses        714      633      -81     

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[RobPasMue]

Awesome work @RyanJWard - thanks for all these tests!