fix: load keys for decryption separately for [Room].searchEvents

[coder-with-a-bushido]

Fix for https://github.com/famedly/product-management/issues/3890

[codecov]

Codecov Report

❌ Patch coverage is 31.03448% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.38%. Comparing base (2ba4922) to head (6900df9).

Files with missing lines	Patch %	Lines
lib/src/room.dart	8.33%	11 Missing ⚠️
lib/encryption/key_manager.dart	47.05%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2334      +/-   ##
==========================================
- Coverage   55.76%   55.38%   -0.38%     
==========================================
  Files         160      160              
  Lines       19850    19867      +17     
==========================================
- Hits        11069    11004      -65     
- Misses       8781     8863      +82     

Files with missing lines	Coverage Δ
lib/encryption/key_manager.dart	86.09% <47.05%> (-0.88%)	⬇️
lib/src/room.dart	75.64% <8.33%> (-0.62%)	⬇️

... and 8 files with indirect coverage changes

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[td-famedly]

@coder-with-a-bushido could use tests