During ContainerBasedSpiller::mergeSpills, the merge-then-write loop
reads from a MergeIterator whose sources are ContainerIterators backed
by WiredTiger cursors. When an insert triggers a WT_ROLLBACK (write
conflict), writeConflictRetry calls rollback_transaction() on the
WiredTiger session. That resets all cursor positions in that session —
including the read cursors held by the ContainerIterators inside
mergeIterator.

On the next retry iteration, mergeIterator resumes from the cursors'
new (reset) position rather than from where the merge left off. Depending
on where the reset lands, elements can be silently dropped or duplicated
in the output spill file.

*Fix*

Collect all merged elements into a std::vector before entering the
write loop. Separating the read phase from the write phase ensures that
cursor resets during write retries cannot affect elements that have
already been consumed from the iterator.

*Steps to reproduce*

Enable the WTWriteConflictException failpoint with nTimes >= 1 and
call mergeSpills with multiple spill iterators. The merged output will
be missing or contain duplicate entries depending on which cursor
position the rollback resets to.

*Tests added*

MergeSpillsRetryOnWriteConflict, MergeSpillsTwoWriteConflictsRetryCount,
MergeSpillsRandomWriteConflicts in container_based_spiller_test.cpp
(ContainerBasedSpillerWTTest fixture).