Fix writeHeader computation in RollingRandomAccessFileManager

The writeHeader boolean was computed after creating the RandomAccessFile, which creates the file if it doesn't exist. This made file.exists() always return true, causing incorrect header write decisions.

This change captures the file's pre-existing state before opening the RandomAccessFile by:

• Declaring a fileExistedBefore boolean initialized to false
• Setting it by checking fileName != null && new File(fileName).exists() before calling new RandomAccessFile(...)
• Using !fileExistedBefore instead of !file.exists() in the writeHeader calculation

When fileName is null, fileExistedBefore remains false as expected.