_checkpoint_db_internal is a large and complex function containing many distinct phases of the checkpoint process. The checkpoint fsync logic is a self-contained piece of code. Leaving it there makes _checkpoint_db_internal harder to read and reason about.

The aim of this ticket is to extract the checkpoint fsync sync block into a new helper _checkpoint_fsync_post, making _checkpoint_db_internal easier to read and reducing its complexity.

Definition of Done:

• checkpoint_fsync_post exists as a static helper in checkpoint_txn.c and is called from _checkpoint_db_internal
• Existing tests pass