Add AvailabilityModel enum for plant availability models

[chris-ashe]

Description

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 68.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.70%. Comparing base (808481e) to head (8e03b83).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/availability.py	75.00%	5 Missing ⚠️
process/core/solver/objectives.py	33.33%	2 Missing ⚠️
process/core/scan.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4468      +/-   ##
==========================================
+ Coverage   48.64%   48.70%   +0.05%     
==========================================
  Files         151      151              
  Lines       29704    29750      +46     
==========================================
+ Hits        14449    14489      +40     
- Misses      15255    15261       +6     

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[timothy-nunn]

I believe you are still missing some places where the switch value is checked:

[timothy-nunn]

Couple of questions about code that seems might be outdated and from when we only had 1 availability model. Otherwise happy with code changes you have made.