Background: WT-17536 implements the in-memory pending key list populated by set_key. At checkpoint time, WiredTiger needs to select the correct key from that list — the entry with the highest timestamp not exceeding the checkpoint's timestamp — and durably write it to the KEK page in v2 format with the timestamp. Entries newer than the checkpoint timestamp must be retained for future checkpoints; older entries can be freed.

Currently the checkpoint always uses the pull-based get_key and writes v1 KEK pages with no timestamp.

Acceptance Criteria:

• At checkpoint time, the correct key is selected from the pending list (highest timestamp <= checkpoint timestamp)
• Selected key is written to the KEK page in v2 format with its timestamp
• Entries newer than the checkpoint timestamp are retained for future checkpoints
• Entries older than the one used are freed after checkpoint completes
• Checkpoint falls back to get_key when v1 KEK page

Depends on WT-17536.