fix(session verification): drop incoming requests we can't complete

[stefanceriu]

A verified session that is missing the private self-signing key can neither sign the other device nor be signed by it, so the request is no longer surfaced to the delegate.

Based off the discussions in element-hq/element-x-ios#6099

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.22%. Comparing base (7def4e6) to head (5a6ba5d).
⚠️ Report is 9 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6971      +/-   ##
==========================================
- Coverage   90.23%   90.22%   -0.01%     
==========================================
  Files         408      408              
  Lines      117281   117281              
  Branches   117281   117281              
==========================================
- Hits       105823   105818       -5     
- Misses       7511     7516       +5     
  Partials     3947     3947              

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing verificationRequestMissingSecrets (5a6ba5d) with main (4fbc1e8)}

[andybalaam]

Looks good, thanks!