Fix #9169 refine idle timeout and failure

Only fail request callback if a failure has not been otherwise notified.
Slight optimization for failing idle timeouts by avoiding double lock.
Always create a failure if failing the callback.

jetty-core/jetty-server/src/main/java/org/eclipse/jetty/server/internal/HttpChannelState.java

@@ -395,13 +395,12 @@ public Runnable onIdleTimeout(TimeoutException t)
                     });
                 }
 
-                // otherwise, we are going to fail the callback, either directly or by a call to onFailure, so ensure all future IO
-                // operations are failed as well
-                _failure = Content.Chunk.from(t, true);
-
-                // if there is no failure listener, then we can fail the callback directly without a double lock
+                // otherwise, if there is no failure listener, then we can fail the callback directly without a double lock
                 if (_onFailure == null && _request != null)
+                {
+                    _failure = Content.Chunk.from(t, true);
                     return () -> _request._callback.failed(t);
+                }
             }
         }