Migrate MediatR integration to Mediator

[neozhu]

Summary

• replace repository MediatR usage with Mediator.Abstractions and wire runtime registration through Mediator.SourceGenerator
• preserve scoped mediator and notification publishing behavior, and remove duplicate pipeline registration during the migration
• add migration design/plan docs and expand unit/integration safety-net coverage for mediator runtime behavior

Test Plan

• dotnet test tests/Application.UnitTests/Application.UnitTests.csproj -v minimal -p:UseSharedCompilation=false
• dotnet test tests/Application.IntegrationTests/Application.IntegrationTests.csproj --filter FullyQualifiedName~MediatorCompatibilityTests -v minimal -p:UseSharedCompilation=false
• manual verification completed locally

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

In general, to fix a “Useless upcast” warning, you remove the unnecessary explicit cast and rely on C#’s implicit conversion from the expression’s type to the target type. This keeps the code simpler and clearer, without changing runtime behavior.

Here, the method Send<TResponse>(IRequest<TResponse> request, ...) returns ValueTask<TResponse>. When the request is a PingQuery, it currently returns new ValueTask<TResponse>((TResponse)(object)"pong"). The inner (object) cast is the useless upcast: "pong" is a string, which already implicitly converts to object. The outer (TResponse) cast is still needed to satisfy the generic return type in this fake/test implementation. So the best fix, without changing any behavior or public surface, is to remove only the (object) cast and leave (TResponse)"pong".

Concretely:

• In tests/Application.UnitTests/Infrastructure/MediatorCompatibility/ScopedMediatorTests.cs, within the FakeMediator.Send<TResponse>(IRequest<TResponse> request, ...) method, edit line 110 to change ((TResponse)(object)"pong") to ((TResponse)"pong").
• No new methods, imports, or definitions are required.