Replace MediatR with source-generated Mediator across application and tests

[Copilot]

This issue replaces runtime MediatR wiring with source-generated Mediator. The change updates dependencies and mediator integration points so command/query/notification flow uses Mediator abstractions and generated registrations.

• Dependency migration

  • Replaced MediatR package usage with:
    • Mediator.Abstractions
    • Mediator.SourceGenerator (host project)
  • Updated central package management in Directory.Packages.props accordingly.

• Mediator API and namespace transition

  • Swapped using MediatR; to using Mediator; in core, infrastructure, web, and functional test code paths.
  • Kept existing mediator interaction patterns (Send, Publish) while moving to Mediator interfaces.

• DI registration update (source-generated setup)

  • Replaced AddMediatR(...) with AddMediator(...).
  • Configured explicit assemblies for handler/message discovery to align with source generation.

• Infrastructure naming alignment

  • Renamed domain event dispatcher from MediatRDomainEventDispatcher to MediatorDomainEventDispatcher.
  • Updated DI registration to use renamed implementation.

Example of the new registration pattern:

builder.Services.AddMediator(options =>
{
	options.Assemblies =
	[
		typeof(IAggregateRoot),
		typeof(AppDbContext),
		typeof(Program)
	];
});

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• api.cdnjs.com
  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/dotnet /usr/share/dotnet/sdk/10.0.201/MSBuild.dll /noautoresponse /nologo /nodemode:1 /nodeReuse:true /low:false (dns block)
  • Triggering command: /usr/bin/dotnet dotnet test tests/DevBetterWeb.FunctionalTests/DevBetterWeb.FunctionalTests.csproj --filter FullyQualifiedName~NoOpMediator (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)