add test for test byPlaceholder children in envelope tags endpoint

[IamDejv]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.91%. Comparing base (959719c) to head (ac550b2).

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x     #367      +/-   ##
============================================
- Coverage     97.95%   97.91%   -0.04%     
  Complexity      512      512              
============================================
  Files            83       83              
  Lines          1367     1199     -168     
============================================
- Hits           1339     1174     -165     
+ Misses           28       25       -3     

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