refactor(acp): split session tests and share rate limiter

src/infra/fixed-window-rate-limit.ts

@@ -0,0 +1,48 @@
+export type FixedWindowRateLimiter = {
+  consume: () => {
+    allowed: boolean;
+    retryAfterMs: number;
+    remaining: number;
+  };
+  reset: () => void;
+};
+
+export function createFixedWindowRateLimiter(params: {
+  maxRequests: number;
+  windowMs: number;
+  now?: () => number;
+}): FixedWindowRateLimiter {
+  const maxRequests = Math.max(1, Math.floor(params.maxRequests));
+  const windowMs = Math.max(1, Math.floor(params.windowMs));
+  const now = params.now ?? Date.now;
+
+  let count = 0;
+  let windowStartMs = 0;
+
+  return {
+    consume() {
+      const nowMs = now();
+      if (nowMs - windowStartMs >= windowMs) {
+        windowStartMs = nowMs;
+        count = 0;
+      }
+      if (count >= maxRequests) {
+        return {
+          allowed: false,
+          retryAfterMs: Math.max(0, windowStartMs + windowMs - nowMs),
+          remaining: 0,
+        };
+      }
+      count += 1;
+      return {
+        allowed: true,
+        retryAfterMs: 0,
+        remaining: Math.max(0, maxRequests - count),
+      };
+    },
+    reset() {
+      count = 0;
+      windowStartMs = 0;
+    },
+  };
+}