[Frontend] Support configurable mm placeholder strings & flexible video sampling policies via CLI flags. (#20105)

Signed-off-by: Chenheli Hua <huachenheli@outlook.com>

tests/entrypoints/openai/test_serving_chat.py

@@ -3,8 +3,8 @@
 
 import asyncio
 from contextlib import suppress
-from dataclasses import dataclass
-from typing import Optional
+from dataclasses import dataclass, field
+from typing import Any, Optional
 from unittest.mock import MagicMock
 
 from vllm.config import MultiModalConfig
@@ -40,6 +40,8 @@ class MockModelConfig:
     allowed_local_media_path: str = ""
     encoder_config = None
     generation_config: str = "auto"
+    media_io_kwargs: dict[str, dict[str, Any]] = field(default_factory=dict)
+    mm_placeholder_str_override: dict[str, str] = field(default_factory=dict)
 
     def get_diff_sampling_param(self):
         return self.diff_sampling_param or {}