Switch to Mediator in Arius5

[woutervanranst]

Summary

• replace MediatR with Mediator library and source generator
• adjust commands, handlers, CLI, and tests for Mediator APIs
• expose core internals to CLI

Testing

• dotnet build
• dotnet test (fails: No valid combination of account information found)

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https://chatgpt.com/codex/tasks/task_b_68a9ac5eaee88324aa30be6270f1a91a

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.07%. Comparing base (ae791ba) to head (d5a6de1).

Additional details and impacted files

@@           Coverage Diff           @@
##           arius5      #55   +/-   ##
=======================================
  Coverage   79.07%   79.07%           
=======================================
  Files          43       43           
  Lines        1625     1625           
  Branches      132      132           
=======================================
  Hits         1285     1285           
  Misses        289      289           
  Partials       51       51           

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