fix(ffi): check that our own device is cross-signed before responding to incoming user verification requests

[stefanceriu]

Receiving incoming user verification requests was possible even if the current device wasn't cross-signed.
This check for that before firing the delegate and prevents us from having to handle it on all clients.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.69%. Comparing base (8d74d46) to head (49e3051).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4650      +/-   ##
==========================================
- Coverage   85.69%   85.69%   -0.01%     
==========================================
  Files         292      292              
  Lines       33562    33562              
==========================================
- Hits        28761    28760       -1     
- Misses       4801     4802       +1     

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