Api add flush cache timeout (#21413)

Signed-off-by: root <wenjun7j@gmail.com>

docs/basic_usage/native_api.ipynb

@@ -185,7 +185,15 @@
    "source": [
     "## Flush Cache\n",
     "\n",
-    "Flush the radix cache. It will be automatically triggered when the model weights are updated by the `/update_weights` API."
+    "Flush the radix cache. It will be automatically triggered when the model weights are updated by the `/update_weights` API.\n",
+    "\n",
+    "Parameters:\n",
+    "- `timeout` (query, float, default `0`, unit: seconds): Wait time for idle state before flushing. `0` means fail fast if not idle. When HiCache async operations are in-flight, a non-zero timeout allows the server to wait until idle before flushing, avoiding unnecessary 400 errors.\n",
+    "\n",
+    "```bash\n",
+    "# With timeout (wait up to 30s for idle state)\n",
+    "curl -s -X POST \"http://127.0.0.1:30000/flush_cache?timeout=30\"\n",
+    "```"
    ]
   },
   {