Read failed-run activity and prefer that run's error over an earlier assistant
reply. Keep successful replies unchanged and retrieve long errors in full
without taking diagnostics from a later run.

Add failed-run, pagination, run-isolation and successful-recovery regressions
for the existing review on pingdotgg/t3code#16719.

Implemented with GPT-6.1-Sol through Codex in T3 Code on behalf of Lenoridev.