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

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

tests/async_engine/test_async_llm_engine.py

@@ -6,8 +6,8 @@ import os
 import uuid
 from asyncio import CancelledError
 from copy import copy
-from dataclasses import dataclass
-from typing import Optional
+from dataclasses import dataclass, field
+from typing import Any, Optional
 
 import pytest
 import pytest_asyncio
@@ -32,6 +32,8 @@ class RequestOutput:
 @dataclass
 class MockModelConfig:
     use_async_output_proc = True
+    media_io_kwargs: dict[str, dict[str, Any]] = field(default_factory=dict)
+    mm_placeholder_str_override: dict[str, str] = field(default_factory=dict)
 
 
 class MockEngine: