Stabilize rate limiter tests by using dependency injection

lib/req/rate_limiter/clock.ex

@@ -0,0 +1,10 @@
+defmodule Req.RateLimiter.Clock do
+  @moduledoc """
+  Behaviour for time operations used by `Req.RateLimiter`.
+
+  Allows injecting a fake clock in tests for deterministic throttle assertions.
+  """
+
+  @callback now() :: integer()
+  @callback sleep(non_neg_integer()) :: :ok
+end