chore: upgrade src-Arius5 projects to .NET 9

[woutervanranst]

Summary

• target .NET 9 across CLI, Core, and test projects in src-Arius5
• update NuGet dependencies, retaining WouterVanRanst.Utils 2.0.4, MediatR 12.5, and FluentAssertions 7.2
• revert SqliteStateDatabaseContext to prior RemovePrefix behavior

Testing

• dotnet restore src-Arius5/Arius.sln
• dotnet list src-Arius5/Arius.sln package --outdated
• dotnet test src-Arius5/Arius.sln (fails: The configuration file 'secrets.json' was not found and is not optional)

---

https://chatgpt.com/codex/tasks/task_b_68a97b1bb9c88324a4f5d69e3ea8ff36

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.01%. Comparing base (4849997) to head (9cc3a81).
⚠️ Report is 1 commits behind head on arius5.

Additional details and impacted files

@@            Coverage Diff             @@
##           arius5      #53      +/-   ##
==========================================
- Coverage   79.07%   79.01%   -0.07%     
==========================================
  Files          43       43              
  Lines        1625     1625              
  Branches      132      132              
==========================================
- Hits         1285     1284       -1     
  Misses        289      289              
- Partials       51       52       +1     

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