fix(server): Limit validation to 50k issues

[nellh]

Prevents failing to report validation for datasets with larger than 16MB BSON issues.

We may remove this limit if the upstream validator provides a less verbose output.

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 46.85%. Comparing base (794f576) to head (ddfd375).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...enneuro-server/src/graphql/resolvers/validation.ts	44.44%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3277      +/-   ##
==========================================
- Coverage   46.85%   46.85%   -0.01%     
==========================================
  Files         610      610              
  Lines       39102    39111       +9     
  Branches     1254     1255       +1     
==========================================
+ Hits        18321    18325       +4     
- Misses      20585    20590       +5     
  Partials      196      196              

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