Fix index serialization when adding manifests

[jonesbusy]

Description

Artifact type was wrongly copied when adding or removing manifests

Added assertion to prevent this issue

Found while debugging #610

Testing done

mvn clean install

Submitter checklist

• I have read and understood the CONTRIBUTING guide
• I have run mvn license:update-file-header, mvn spotless:apply, pre-commit run -a, mvn clean install before opening the PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.92%. Comparing base (395122f) to head (2b1fd13).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #615      +/-   ##
============================================
+ Coverage     87.26%   87.92%   +0.65%     
  Complexity      846      846              
============================================
  Files            42       42              
  Lines          2553     2551       -2     
  Branches        317      317              
============================================
+ Hits           2228     2243      +15     
+ Misses          202      185      -17     
  Partials        123      123              

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