The FN-1598 merger.ts async conversion introduced an escalation strategy where
attempt 3 uses -X theirs and succeeds when AI fails. The tests were expecting
the merge to fail after 3 AI attempts, but now attempt 3 succeeds via the
theirs strategy.
This commit fixes the test mocks to make the -X theirs merge fail so all
3 attempts exhaust and the function throws as expected.
88b3ae4ad6