fix(active-memory): stop caller timeouts from continuing failover (#65046)

* fix(active-memory): remove built-in fallback model

* fix(active-memory): wire external abort through failover

extensions/codex/src/app-server/event-projector.ts

@@ -153,6 +153,7 @@ export class CodexAppServerEventProjector {
       (turnFailed ? (this.completedTurn?.error?.message ?? "codex app-server turn failed") : null);
     return {
       aborted: this.aborted || turnInterrupted,
+      externalAbort: false,
       timedOut: false,
       idleTimedOut: false,
       timedOutDuringCompaction: false,