test(gateway): isolate cache proof from runtime startup

Amp-Thread-ID: https://ampcode.com/threads/T-019fee8d-665d-707b-a380-23f2a6a1ce03
Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/gateway/session-utils.perf.test.ts

@@ -66,6 +66,16 @@ describe("listSessionsFromStore resolver cache", () => {
           model: tuple.model,
           inputTokens: 100,
           outputTokens: 50,
+          // Keep runtime selection outside this cache proof. ACP metadata gives
+          // the row a concrete runtime without cold-loading provider plugins.
+          acp: {
+            backend: "acpx",
+            agent: "codex",
+            runtimeSessionName: `cache-proof-${i}`,
+            mode: "oneshot",
+            state: "idle",
+            lastActivityAt: now,
+          },
         } as SessionEntry;
       }