Add test to prevent premature removal of conflict-relevant data.

Add test to prevent premature removal of conflict-relevant data.

A test has been added to ensure that conflict-relevant data is not
prematurely removed when a concurrent prepared transaction is being
committed on the publisher.

This test introduces an injection point that simulates the presence of a
prepared transaction in the commit phase, validating that the system
correctly delays conflict slot advancement until the transaction is fully
committed.

Additionally, the test serves as a safeguard for developers, ensuring that
the acquisition of the commit timestamp does not occur before marking
DELAY_CHKPT_IN_COMMIT in RecordTransactionCommitPrepared.

Reported-by: Robert Haas <robertmhaas@gmail.com>
Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/OS9PR01MB16913F67856B0DA2A909788129400A@OS9PR01MB16913.jpnprd01.prod.outlook.com