fix: preserve concurrent prefix updates

[vycdev2]

Summary

• prevent an in-flight prefix database lookup from overwriting a newer cached prefix set by an administrator
• add a deterministic regression test that pauses the database query while the cache is updated

Verification

• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --filter FullyQualifiedName~GuildPrefixServiceTests --logger 'console;verbosity=minimal' — passed: 1/1 (the regression failed against the unchanged implementation with old! instead of new!)
• dotnet build — passed with 0 errors; 2 existing NU1903 warnings for SQLitePCLRaw.lib.e_sqlite3 2.1.6
• dotnet test --no-build --logger 'console;verbosity=minimal' — passed: 429, skipped: 1, failed: 0
• git diff --check — passed

Risk

• Low: the cache now keeps an entry written while an asynchronous database lookup is in flight; normal cache misses and explicit updates retain their existing behavior.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.