Propagate the cause that led Reactor"s debug agent to fail

Prior to this commit, DebugAgentEnvironmentPostProcessor throws a
RuntimeException with a generic error message if its initialization
fails. The causing exception is discarded, which makes error analysis
unnecessarily difficult.

This commit attaches the cause to the newly thrown RuntimeException.

See gh-22777

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/reactor/DebugAgentEnvironmentPostProcessor.java

@@ -49,7 +49,7 @@ public class DebugAgentEnvironmentPostProcessor implements EnvironmentPostProces
 					debugAgent.getMethod("init").invoke(null);
 				}
 				catch (Exception ex) {
-					throw new RuntimeException("Failed to init Reactor's debug agent");
+					throw new RuntimeException("Failed to init Reactor's debug agent", ex);
 				}
 			}
 		}