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/utils.py

@@ -0,0 +1,41 @@
+import json
+import sys
+import time
+from pathlib import Path
+
+
+def eprint(*args, **kwargs):
+    print(*args, file=sys.stderr, **kwargs)
+
+
+def load_json_arg(value, name):
+    if value is None:
+        return None
+    if isinstance(value, dict):
+        return value
+    if isinstance(value, str) and value.startswith("@"):
+        path = Path(value[1:])
+        try:
+            return json.loads(path.read_text(encoding="utf-8"))
+        except Exception as exc:
+            raise ValueError(f"Failed to read {name} from {path}: {exc}") from exc
+    try:
+        return json.loads(value)
+    except Exception as exc:
+        raise ValueError(f"Invalid JSON for {name}: {exc}") from exc
+
+
+def split_csv(value):
+    if value is None:
+        return None
+    if isinstance(value, list):
+        return value
+    if isinstance(value, str):
+        items = [item.strip() for item in value.split(",")]
+        return [item for item in items if item]
+    return [value]
+
+
+def unique_name(prefix):
+    return f"{prefix}_{int(time.time() * 1000)}"
+