fix: evict expired SESSION_MANAGER_CACHE entries on TTL miss

isSessionManagerCached() checks TTL before returning stale hits but
never deletes expired entries from the Map. They accumulate
indefinitely over the lifetime of a long-running gateway.

Delete the expired entry when the TTL check fails so the Map stays
bounded to active sessions.

Closes #51820

src/agents/pi-embedded-runner/session-manager-cache.ts

@@ -42,7 +42,11 @@ function isSessionManagerCached(sessionFile: string): boolean {
   }
   const now = Date.now();
   const ttl = getSessionManagerTtl();
-  return now - entry.loadedAt <= ttl;
+  if (now - entry.loadedAt > ttl) {
+    SESSION_MANAGER_CACHE.delete(sessionFile);
+    return false;
+  }
+  return true;
 }
 
 export async function prewarmSessionFile(sessionFile: string): Promise<void> {