In _checkpoint_db_internal, the block responsible for checkpointing individual trees - injecting timing stress, calling _checkpoint_apply_to_dhandles, and recording tree checkpoint duration statistics - is self-contained and does not interact with the surrounding code. Leaving it inline adds unnecessary length and noise to the function's main flow.

The aim of this ticket is to extract this block into a new static helper _checkpoint_selected_dhandles making _checkpoint_db_internal improving code readability and reducing complexity.

Definition of Done

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