docs: add rate limit redis smoke test

docs/rate_limit_redis.md

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+# Rate Limit Redis Smoke Test
+
+This note records the local Redis setup and a quick smoke test for rate-limit storage.
+
+## Docker setup (local)
+
+Image source used in this environment:
+- `docker.hub.ipao.vip/redis:7`
+
+Start container (password enabled):
+
+```bash
+sudo -n docker run -d --name quyun-redis -p 6379:6379 docker.hub.ipao.vip/redis:7 \
+  redis-server --requirepass testpass
+```
+
+## Config sample
+
+```toml
+[Http.RateLimit]
+Enabled = true
+Max = 120
+WindowSeconds = 60
+Message = "Too Many Requests"
+
+[Http.RateLimit.Redis]
+Addrs = ["127.0.0.1:6379"]
+Username = ""
+Password = "testpass"
+DB = 2
+Prefix = "rl:"
+```
+
+## Smoke test commands
+
+```bash
+sudo -n docker exec quyun-redis redis-cli ping
+sudo -n docker exec quyun-redis redis-cli -a testpass ping
+sudo -n docker exec quyun-redis redis-cli -a testpass -n 2 set rl:tenant:demo 1 EX 60
+sudo -n docker exec quyun-redis redis-cli -a testpass -n 2 get rl:tenant:demo
+sudo -n docker exec quyun-redis redis-cli -a testpass -n 0 get rl:tenant:demo
+```
+
+Expected behavior:
+- First ping returns `NOAUTH Authentication required.`
+- Authenticated ping returns `PONG`.
+- DB 2 contains `rl:tenant:demo = 1`, DB 0 does not.
+
+## Cleanup
+
+```bash
+sudo -n docker rm -f quyun-redis
+```