feat(webhooks): validate Freight during creation

[hiddeco]

This adds more validation logic for the creation of Freight, ensuring that Freight contains precisely one artifact for each subscription of the Warehouse it references.

Fixes #2117

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[codecov]

Codecov Report

Attention: Patch coverage is 98.60140% with 2 lines in your changes missing coverage. Please review.

Project coverage is 46.56%. Comparing base (086477e) to head (54a603b).
Report is 8 commits behind head on main.

Files	Patch %	Lines
internal/webhook/freight/webhook.go	98.60%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2118      +/-   ##
==========================================
+ Coverage   45.93%   46.56%   +0.62%     
==========================================
  Files         238      239       +1     
  Lines       16542    16741     +199     
==========================================
+ Hits         7599     7795     +196     
- Misses       8571     8573       +2     
- Partials      372      373       +1     

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