fix(backends): VKAppOAuth2 now requires auth_key

[nijel]

Tighten the VK app backend key validation to reject payload without signature.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.56%. Comparing base (06e154a) to head (e0737ec).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1811      +/-   ##
==========================================
+ Coverage   84.41%   84.56%   +0.14%     
==========================================
  Files         342      342              
  Lines       12593    12637      +44     
  Branches      607      608       +1     
==========================================
+ Hits        10630    10686      +56     
+ Misses       1742     1727      -15     
- Partials      221      224       +3     

Flag	Coverage Δ
unittests	84.56% <100.00%> (+0.14%)	⬆️

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