Mark RetryException#getCause non null

This commit simplifies RetryException to always require a root cause
and mark it as not nullable. Such exception is the exception thrown by
the retryable operation and should always be available as it explains
why the invocation was a candidate for retrying in the first place.

Closes gh-35332

spring-context/src/main/java/org/springframework/resilience/retry/AbstractRetryInterceptor.java

@@ -112,8 +112,7 @@ public abstract class AbstractRetryInterceptor implements MethodInterceptor {
 			});
 		}
 		catch (RetryException ex) {
-			Throwable cause = ex.getCause();
-			throw (cause != null ? cause : new IllegalStateException(ex.getMessage(), ex));
+			throw ex.getCause();
 		}
 	}