[Core] Support reseting all running requests' KV while calling reset_prefix_cache (#28827)

Signed-off-by: Zhuohan Li <zhuohan123@gmail.com>
Signed-off-by: Nick Hill <nhill@redhat.com>
Co-authored-by: Nick Hill <nhill@redhat.com>

vllm/engine/protocol.py

@@ -116,7 +116,7 @@ class EngineClient(ABC):
         ...
 
     @abstractmethod
-    async def reset_prefix_cache(self) -> None:
+    async def reset_prefix_cache(self, reset_running_requests: bool = False) -> bool:
         """Reset the prefix cache"""
         ...