Stabilize rate limiter tests by using dependency injection

test/support/fake_clock.ex

@@ -0,0 +1,34 @@
+defmodule Req.RateLimiter.FakeClock do
+  @moduledoc """
+  Fake clock for deterministic rate limiter tests.
+
+  Stores the current time in the process dictionary so each test process
+  has its own isolated clock state. Works with `async: true`.
+  """
+
+  @behaviour Req.RateLimiter.Clock
+
+  @key :fake_clock_now
+
+  @spec set(integer()) :: :ok
+  def set(now) do
+    Process.put(@key, now)
+    :ok
+  end
+
+  @spec advance(non_neg_integer()) :: :ok
+  def advance(ms) do
+    Process.put(@key, now() + ms)
+    :ok
+  end
+
+  @impl true
+  def now do
+    Process.get(@key, 0)
+  end
+
+  @impl true
+  def sleep(ms) do
+    advance(ms)
+  end
+end