Fixes for tests

[MJGaughran]

These are the code changes that were made for the improved suite of tests. This includes:

• Removal of dead code (aiming for high coverage)
• Renaming confusing variables
• Use of consistent error handling
• Fixing a couple bugs

Each type of fix is covered by a separate commit.

[codecov]

Codecov Report

❌ Patch coverage is 54.54545% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.53%. Comparing base (61d4864) to head (6740295).

Files with missing lines	Patch %	Lines
src/deploy_tools/snapshot.py	41.17%	10 Missing ⚠️
src/deploy_tools/compare.py	0.00%	9 Missing ⚠️
src/deploy_tools/models/save_and_load.py	66.66%	2 Missing ⚠️
src/deploy_tools/sync.py	88.88%	2 Missing ⚠️
src/deploy_tools/deploy.py	66.66%	1 Missing ⚠️
src/deploy_tools/validate.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
- Coverage   83.59%   82.53%   -1.07%     
==========================================
  Files          27       27              
  Lines         957      979      +22     
==========================================
+ Hits          800      808       +8     
- Misses        157      171      +14     

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[ptsOSL]

Looks good