test: simplify bootstrap cache hit count

src/agents/bootstrap-budget.test.ts

@@ -475,7 +475,12 @@ describe("bootstrap prompt warnings", () => {
       injectLegacyWarning(optimizedTurns[2] ?? "", warningLines),
     ];
     const cacheHitRate = (turns: string[]) => {
-      const hits = turns.slice(1).filter((turn, index) => turn === turns[index]).length;
+      let hits = 0;
+      for (let index = 1; index < turns.length; index++) {
+        if (turns[index] === turns[index - 1]) {
+          hits++;
+        }
+      }
       return hits / Math.max(1, turns.length - 1);
     };