common/buf/writer.go: Fix buffered writes failing when payload exceeds remaining capacity (#6723)

Fixes https://github.com/XTLS/Xray-core/issues/5287

common/buf/writer.go

@@ -118,7 +118,9 @@ func (w *BufferedWriter) Write(b []byte) (int, error) {
 
 		nBytes, err := w.buffer.Write(b)
 		totalBytes += nBytes
-		if err != nil {
+
+		// ErrBufferFull means a partial write, so flush below and continue
+		if err != nil && err != ErrBufferFull {
 			return totalBytes, err
 		}
 		if !w.buffered || w.buffer.IsFull() {