Feat: Benchmark CLI additions and documentation (#12536)

### What problem does this PR solve?

This PR adds a dedicated HTTP benchmark CLI for RAGFlow chat and
retrieval endpoints so we can measure latency/QPS.

### Type of change

- [x] Documentation Update
- [x] Other (please describe): Adds a CLI benchmarking tool for
chat/retrieval latency/QPS

---------

Co-authored-by: Liu An <asiro@qq.com>

test/benchmark/retrieval.py

@@ -0,0 +1,39 @@
+import time
+from typing import Any, Dict, List, Optional
+
+from .http_client import HttpClient
+from .metrics import RetrievalSample
+
+
+class RetrievalError(RuntimeError):
+    pass
+
+
+def build_payload(
+    question: str,
+    dataset_ids: List[str],
+    document_ids: Optional[List[str]] = None,
+    payload: Optional[Dict[str, Any]] = None,
+) -> Dict[str, Any]:
+    body = dict(payload or {})
+    if "question" not in body:
+        body["question"] = question
+    if "dataset_ids" not in body:
+        body["dataset_ids"] = dataset_ids
+    if document_ids is not None and "document_ids" not in body:
+        body["document_ids"] = document_ids
+    return body
+
+
+def run_retrieval(client: HttpClient, payload: Dict[str, Any]) -> RetrievalSample:
+    t0 = time.perf_counter()
+    response = client.request("POST", "/retrieval", json_body=payload, stream=False)
+    raw = response.content
+    t1 = time.perf_counter()
+    try:
+        res = client.parse_json_bytes(raw)
+    except Exception as exc:
+        return RetrievalSample(t0=t0, t1=t1, error=f"Invalid JSON response: {exc}")
+    if res.get("code") != 0:
+        return RetrievalSample(t0=t0, t1=t1, error=res.get("message"), response=res)
+    return RetrievalSample(t0=t0, t1=t1, error=None, response=res)