2703 getter setter

[rorymckinley]

Description

This is the first of a series of PRs to cleanup usage of Snapshot.get_or_create_latest_for. This functionality was widely used to provide coverage as Lightning migrated to snapshots but is now superfluous for many of the use cases as Workflow creation or modification results in Snapshot creation.

As some of this PR includes updating test fixtures that were relying on the or_create functionality, I have used distinct commits to make it more digestible.

#2703

Validation steps

This functionality was superfluous so there will be no outwardly visible change. The best validation is probably to confirm that my logic as to the removal of the 'or_create' portion of the functionality is correct.

AI Usage

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• Optimisation / refactoring
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• I have not used AI

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Pre-submission checklist

• I have performed a self-review of my code.
• I have implemented and tested all related authorization policies. (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.15%. Comparing base (19bd2bf) to head (28c486d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2716      +/-   ##
==========================================
+ Coverage   91.11%   91.15%   +0.04%     
==========================================
  Files         333      333              
  Lines       11794    11795       +1     
==========================================
+ Hits        10746    10752       +6     
+ Misses       1048     1043       -5     

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