Add trait for `BasePrefixCache` (#12436)

python/sglang/srt/mem_cache/base_prefix_cache.py

+from __future__ import annotations
+
 from abc import ABC, abstractmethod
-from typing import TYPE_CHECKING, Any, NamedTuple, Optional, Tuple
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    NamedTuple,
+    Optional,
+    Protocol,
+    Tuple,
+    runtime_checkable,
+)
 
 import torch
 
+from sglang.srt.mem_cache.allocator import BaseTokenToKVPoolAllocator
+from sglang.srt.mem_cache.memory_pool import ReqToTokenPool
+
 if TYPE_CHECKING:
     from sglang.srt.managers.schedule_batch import Req
-else:
-    Req = Any  # Placeholder for Req type when not type checking
+
+
+@runtime_checkable
+class PrefixCacheTrait(Protocol):
+    req_to_token_pool: ReqToTokenPool
+    token_to_kv_pool_allocator: BaseTokenToKVPoolAllocator
+    page_size: int
+    disable: bool
 
 
 class MatchResult(NamedTuple):
@@ -28,7 +47,7 @@ class MatchResult(NamedTuple):
     host_hit_length: int = 0
 
 
-class BasePrefixCache(ABC):
+class BasePrefixCache(ABC, PrefixCacheTrait):
     """Cache can be indexed by either rid or key."""
 
     @abstractmethod