Simplify CheckLicense to use GetRequiredService instead of fallback constructors

[Copilot]

CheckLicense was using GetService with ?? new LicenseAccessor(...) / ?? new LicenseValidator(...) fallbacks, duplicating construction logic already handled by ServiceRegistrar.AddRequiredServices.

Changes

• MediatRServiceCollectionExtensions.cs: Replace GetService + fallback pattern with direct GetRequiredService<LicenseAccessor>() and GetRequiredService<LicenseValidator>().

// Before
var licenseAccessor = serviceProvider.GetService<LicenseAccessor>() ?? new LicenseAccessor(
    serviceProvider.GetRequiredService<MediatRServiceConfiguration>(),
    serviceProvider.GetRequiredService<ILoggerFactory>()
);
var licenseValidator = serviceProvider.GetService<LicenseValidator>()
                       ?? new LicenseValidator(serviceProvider.GetRequiredService<ILoggerFactory>());

// After
var licenseAccessor = serviceProvider.GetRequiredService<LicenseAccessor>();
var licenseValidator = serviceProvider.GetRequiredService<LicenseValidator>();

• ServiceFactoryTests.cs: Updated the manual registration test to explicitly register LicenseAccessor and LicenseValidator, which are now required in the container.

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