refactor: 设计模式优化 inference 模块导入结构

- 新建 cache.py：SlotAllocator 对象池 + PrefixCacheManager

- 新建 sampling.py：Temperature/TopK/TopP 可组合策略

- TaskStatus 改用 Enum，GenerationParams 值对象模式

- _STOP 移至 cache.py，解除 engine→scheduler 轻量耦合

- 更新测试导入路径，ruff 格式检查通过

scripts/demo/stream_chat.py

@@ -15,7 +15,7 @@ def chat():
     tokenizer = AutoTokenizer.from_pretrained(PARAMETER_ROOT)
     model.to(device="cuda", dtype=torch.bfloat16)
 
-    messages = []
+    messages = [{"role": "system", "content": "You are a helpful assistant."}]
     engine = InferenceEngine(model=model, tokenizer=tokenizer)
 
     while True: