Validate that arms only have one shield

[pavelbraginskiy]

Fixes #6622.

A Mek with multiple shields in one arm should be considered invalid.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.17%. Comparing base (2f7d753) to head (14ef21f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6623      +/-   ##
============================================
+ Coverage     29.16%   29.17%   +0.01%     
- Complexity    15427    15449      +22     
============================================
  Files          2865     2865              
  Lines        280027   280034       +7     
  Branches      49307    49310       +3     
============================================
+ Hits          81667    81709      +42     
+ Misses       192884   192842      -42     
- Partials       5476     5483       +7     

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