Create system test

[callumforrester]

Create system test proving that blueapi and the nexus writer work as expected

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (fe7e3d7) to head (219b7af).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #3   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           12        12           
=========================================
  Hits            12        12           

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[GDYendell]

Looks good. Spotted some typos. Couple of questions to make sure I understand what this is doing.

[callumforrester]

@GDYendell any strong opinions on squash merge/rebase merge for this repo, I'm easy.

I also got rid of the empty plans/__init__.py file, which was a vestige of a previous version, this can be replaced by your changes in #4

[GDYendell]

These commits are neat so I would do a merge commit, but in general I don't mind using squash commits if we want it to be consistent if it means not merging review change commits.