test_verify_disagg fails in the Ubuntu 20.04 ASan unit-test task. WT_SESSION.verify returns EINVAL because a cell’s durable start timestamp is 28226, but the stable timestamp used for validation is 0 and the parent aggregate time window is empty.

Neither the server nor WiredTiger currently sets the stable timestamp on this path.

WT-18771 fixed a similar failure for pages reconstructed from a base image and deltas by using the checkpoint timestamp instead of the verifying connection’s live stable timestamp. That fix does not cover this failing path.

Failure logs:
https://spruce.corp.mongodb.com/task/wiredtiger_ubuntu2004_asan_unit_test_xsan_bucket17_9d36b5fde6a45e1f4f63549ffea23a5b09acaaef_26_10_26_00_29_04/logs?execution=0

Error at 2026-10-26 12:52:28.507:
file:test_verify_disagg.wt_stable/WiredTigerCheckpoint.1, WT_SESSION.verify:
__time_value_validate_parent_stable, line 390:
time window has a durable start time after the stable point with an empty parent aggregate time window;
stable time (0, 0), time window start: durable_timestamp=(0, 28226) timestamp=(0, 28226)
prepare_timestamp=(0, 0) prepared_id=0 transaction=0
stop: durable_timestamp=(0, 0) timestamp=([4294967295|tel:4294967295], 4294967295)
prepare_timestamp=(0, 0) prepared_id=0 transaction=18446744073709551605:
Invalid argument

Expected behaviour:
Verification should succeed for valid disaggregated checkpoint data without failing solely because the live stable timestamp is unset.

Work required:

• Identify the verification path and timestamp source not covered by WT-18771.
• Fix timestamp initialization or use the appropriate checkpoint timestamp for validation.
• Add regression coverage for this path while preserving checks for genuinely invalid time windows.