Fix FlinkJobManagerUrl to use discovered dynamic port

FlinkJobManagerUrl was hardcoded to http://localhost:8081/ but Aspire uses dynamic ports. Changed to use the discovered flinkRestApi value which contains the correct dynamic port mapping.

This fixes the Flink health check failures in Day01 exercises.

Co-authored-by: devstress <30769729+devstress@users.noreply.github.com>

Author: Copilot

LearningCourse/LearningCourse.IntegrationTests/LearningCourseTestBase.cs

@@ -392,7 +392,7 @@ private static async Task WaitForInfrastructureReadyAsync(DateTime appHostStartT
                 GrafanaHostEndpoint = grafanaEndpoint;
                 FlinkRestApiEndpoint = flinkRestApi;
                 FlinkJobGatewayUrl = "http://localhost:8080/";  // Fixed port for JobGateway (not dynamic)
-                FlinkJobManagerUrl = "http://localhost:8081/";  // Fixed port for JobManager (configured in Aspire)
+                FlinkJobManagerUrl = flinkRestApi;  // Use discovered dynamic port from Aspire
 
                 var savedTime = (maxWait - stopwatch.Elapsed).TotalSeconds;
                 TestContext.WriteLine($"✅ All required infrastructure ready after {stopwatch.Elapsed.TotalSeconds:F1}s (saved {savedTime:F1}s with optimized polling)");