Fix Mediator v3 interface breaking changes: ValueTask return types and source generator leak

[Copilot]

CI failing due to two distinct breakages from Mediator v3.0.2 adoption.

NoOpMediator interface mismatch (CS0535, CS0738)

Mediator v3 changed IMediator/ISender/IPublisher to use ValueTask instead of Task, added ICommand<T>/IQuery<T> overloads to ISender, and removed Send<TRequest>(TRequest) where TRequest : IRequest. Updated NoOpMediator to match:

// Before
public Task<TResponse> Send<TResponse>(IRequest<TResponse> request, ...) => Task.FromResult<TResponse>(default);
public Task Publish(object notification, ...) => Task.CompletedTask;

// After
public ValueTask<TResponse> Send<TResponse>(IRequest<TResponse> request, ...) => ValueTask.FromResult<TResponse>(default!);
public ValueTask<TResponse> Send<TResponse>(ICommand<TResponse> request, ...) => ValueTask.FromResult<TResponse>(default!);
public ValueTask<TResponse> Send<TResponse>(IQuery<TResponse> request, ...) => ValueTask.FromResult<TResponse>(default!);
public ValueTask Publish(object notification, ...) => ValueTask.CompletedTask;

Mediator.SourceGenerator transitive leak (CS0436)

Mediator.SourceGenerator in DevBetterWeb.Web.csproj lacked PrivateAssets>all, causing the source generator to run in every test project referencing Web. This re-generated AssemblyReference, MediatorOptions, and Mediator types that conflicted with those already compiled into the Web assembly.

<!-- Before -->
<PackageReference Include="Mediator.SourceGenerator" />

<!-- After -->
<PackageReference Include="Mediator.SourceGenerator">
    <PrivateAssets>all</PrivateAssets>
    <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
</PackageReference>