DirectIO WAL Write does not honor DBOptions.use_fsync

In my test using direct io for WAL write (with ext4 in the kernel), I noticed direct io WAL writes do not do metadata sync even when I have DBOptions.use_fsync = true. I understand that the actual data is persistent with direct IO, but the metadata is not flushed. I think we may be exposed to metadata loss in the case of sudden power outage? Please correct me if otherwise.

In the code, the function WritableFileWriter::Sync(bool use_fsync) is excluding SyncInternal for directio.

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### Expected behavior

### Actual behavior

### Steps to reproduce the behavior

Start at WritableFileWriter::Sync(bool use_fsync) and trace why direct I/O excludes SyncInternal. Compare the expected metadata-sync behavior with the existing direct-I/O WAL path, then define regression coverage showing that DBOptions.use_fsync is honored during WAL writes.

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