fix(server): serialize nested dict config values as JSON (#28094)

Signed-off-by: Ting Sun <suntcrick@gmail.com>

python/sglang/srt/server_args_config_parser.py

@@ -4,6 +4,7 @@ Handles merging of YAML configuration files with command-line arguments.
 """
 
 import argparse
+import json
 import logging
 from pathlib import Path
 from typing import Any, Dict, List
@@ -151,6 +152,8 @@ class ConfigArgumentMerger:
                 self._add_boolean_arg(args, key, value)
             elif isinstance(value, list):
                 self._add_list_arg(args, key, value)
+            elif isinstance(value, dict):
+                self._add_scalar_arg(args, key, json.dumps(value))
             else:
                 self._add_scalar_arg(args, key, value)