Fix missing lastOutputAt in HeartbeatRun test mock

Add lastOutputAt: null to the makeRun helper in inbox.test.ts to match
the updated HeartbeatRun type.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/lib/inbox.test.ts

@@ -113,6 +113,7 @@ function makeRun(id: string, status: HeartbeatRun["status"], createdAt: string,
     externalRunId: null,
     processPid: null,
     processStartedAt: null,
+    lastOutputAt: null,
     retryOfRunId: null,
     processLossRetryCount: 0,
     stdoutExcerpt: null,