Fix writeHeader computation in RollingRandomAccessFileManager

[NeatGuyCoding]

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.

[github-actions]

Job	Requested goals	Build Tool Version	Build Outcome	Build Scan®
build-macos-latest	clean install	3.9.8	✅
build-ubuntu-latest	clean install	3.9.8	✅
build-windows-latest	clean install	3.9.8	✅

Generated by gradle/develocity-actions

[vy]

@NeatGuyCoding, the report and the fix look legitimate, thanks. Would you mind providing also a changelog entry and a test, please?

I'd appreciate it if you can keep the test as minimal as possible.

[NeatGuyCoding]

@vy done