fix: Prevent Docker-in-Docker container startup in GitHub Actions CI

- Fail fast in CI if LocalStack service container is not detected
- Only start Testcontainers in local development environments
- Avoids Docker networking errors in GitHub Actions runners
- Tests now exclusively use the service container in CI

This resolves the 'failed to set up container networking' error by
ensuring tests never attempt to start their own containers when
running in GitHub Actions, where LocalStack is provided as a service.

Author: NinjaRocks

tests/SourceFlow.Cloud.AWS.Tests/TestHelpers/LocalStackTestFixture.cs

@@ -110,9 +110,20 @@ public async Task InitializeAsync()
 
         if (!isAlreadyRunning)
         {
-            Console.WriteLine("Starting new LocalStack container...");
+            // In GitHub Actions, we expect LocalStack to be provided as a service container
+            // If it's not detected, fail fast rather than trying to start a new container
+            if (isGitHubActions)
+            {
+                string errorMessage = "LocalStack service container not detected in GitHub Actions CI. " +
+                    "Ensure the workflow has a 'services.localstack' configuration. " +
+                    "Tests cannot start their own containers in CI due to Docker-in-Docker limitations.";
+                Console.WriteLine($"ERROR: {errorMessage}");
+                throw new InvalidOperationException(errorMessage);
+            }
+            
+            Console.WriteLine("Starting new LocalStack container for local development...");
 
-            // Create LocalStack container
+            // Create LocalStack container (local development only)
             _localStackContainer = new ContainerBuilder()
                 .WithImage("localstack/localstack:latest")
                 .WithPortBinding(4566, 4566)
@@ -126,10 +137,9 @@ public async Task InitializeAsync()
             await _localStackContainer.StartAsync();
             Console.WriteLine("LocalStack container started successfully");
 
-            // Wait for services to be ready - longer delay in CI environments
-            int postStartDelay = isGitHubActions ? 5000 : 2000;
-            Console.WriteLine($"Waiting {postStartDelay}ms for LocalStack services to initialize...");
-            await Task.Delay(postStartDelay);
+            // Wait for services to be ready
+            Console.WriteLine("Waiting 2000ms for LocalStack services to initialize...");
+            await Task.Delay(2000);
         }
 
         // Create AWS clients configured for LocalStack