__checkpoint_db_internal contains a post-commit block responsible for checkpointing and syncing the metadata file. This block mixes concerns - isolation switching, disaggregated shared metadata handling, metadata file checkpoint, metadata sync, and clearing the checkpoint-pinned flag.

The aim of this ticket is to extract this block into a new static helper, to reduce the code complexity and improve readability.

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