Availability and Build Model refactor

[je-cook]

Description

Tackles some more of the remaining quality improvements. At this point the remainder might have to be ignored.

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 35.08772% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.59%. Comparing base (8fe9306) to head (2950a08).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/availability.py	35.08%	37 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4522      +/-   ##
==========================================
+ Coverage   49.30%   49.59%   +0.28%     
==========================================
  Files         150      150              
  Lines       29769    29673      -96     
==========================================
+ Hits        14679    14715      +36     
+ Misses      15090    14958     -132     

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