feat(test): Add Testcontainers for Radarr and Sonarr integration testing

[maxnatamo]

Overview

Adds testing layers and fixtures for testing against real Radarr and Sonarr instances, using Testcontainers and Docker.

This allows for end-to-end testing of adding/removing items from *arr instances. A small integration example:

namespace Fetcharr.SomeIntegrationTests
{
    [IntegrationTest]
    [Collection(nameof(RadarrIntegrationLayer))]
    public class RadarrIntegrationTests(
        RadarrIntegrationLayer layer)
    {
        [Fact]
        public async Task AssertHealthyRadarrInstance()
        {
            // Arrange

            // Act
            IFlurlResponse response = await layer.RadarrApiClient.Request("/api/v3/health").GetAsync();

            // Assert
            response.StatusCode.Should().Be(200);
        }
    }
}

PR Checklist

• Successful Docker build (docker buildx build .)