fix(acpx): record turn activity at the moment a stop fires

Sandbox claude_local runs that hit the 4h wall-clock backstop report
acpLastEventAgeMs at finalization, after the timeout's cancel. Some agents
emit closing events on cancel, so a turn that went silent minutes into the
run can look active at its deadline.

Record, for a stopped turn, the last-event age and the oldest pending
tool's age as of the stop, plus the pending tools' closed ACP ToolKind
labels. The server forwards these to run-failure diagnostics only when
they are non-negative integers or closed enum labels.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

doc/observability.md

@@ -638,6 +638,15 @@ pending. Recent events do not prove useful progress. These fields contain only
 numbers and a boolean, never tool names, IDs, arguments, or event content, and
 do not change the execution timeout, cancellation, or recovery policy.
 
+A stopped turn (wall-clock timeout or operator stop) also records
+`acpLastEventAgeAtStopMs` and `acpOldestPendingToolAgeAtStopMs`, measured at the
+moment the stop fired. A cancel can make the agent emit closing events, so the
+finalization age alone cannot separate a turn that went silent long before its
+deadline from one that was still working. When tools are pending,
+`acpPendingToolKinds` lists their ACP tool kinds as a sorted, comma-separated
+string drawn only from the closed ACP `ToolKind` enum plus `unknown`; the
+server drops the field if any other value appears.
+
 ACP and legacy OpenCode results retain the adapter's resolved wall-clock timeout as
 `adapterExecutionTimeout` in the instance run result. Finalization uses it for
 `effectiveTimeoutSec`, `timeoutSource`, and `timeoutConfigured`. Sources are