refactor(aibox): make profile reasoning_enabled a constructor param (M3)

Profile-level reasoning_enabled was attached as an instance attribute
after construction only when a profile matched; the base class never
declared it and consumers probed with double getattr. Any code that
accidentally set client.reasoning_enabled would silently override the
global switch.

The flag is now an explicit LLMClient constructor parameter with
three-state semantics: True/False (profile-level explicit, or the
global api.reasoning_enabled value concretized when the profile omits
the field) and None (no profile -> consumer falls back to global).
create_llm_client passes it at construction and no longer mutates the
client afterwards; DeepSeekChat._reasoning_enabled simplifies to a
single getattr probe that still tolerates clients without the attr.

Tests: TestM3ReasoningConstructor (4 cases) covering profile True,
profile-omitted -> global concretized, base default None, and the
None -> global fallback at the consumer; one legacy assertion updated
to the new contract (attribute exists, value None). Full suite 299
passed.

aibox/aibox.py

@@ -976,10 +976,10 @@ class DeepSeekChat:
                 pass
     
     def _reasoning_enabled(self) -> bool:
-        """思考模式开关：客户端带档案级 reasoning_enabled 属性时优先（多模型档案），否则用全局配置"""
-        return getattr(self.client, "reasoning_enabled", None) \
-            if getattr(self.client, "reasoning_enabled", None) is not None \
-            else constants.REASONING_ENABLED
+        """思考模式开关：client.reasoning_enabled 非 None 时优先（档案级构造参数），
+        否则回退全局配置；getattr 兼容未实现该契约的自定义 client"""
+        val = getattr(self.client, "reasoning_enabled", None)
+        return val if val is not None else constants.REASONING_ENABLED
 
     def _is_cancelled(self) -> bool:
         """检查是否已被取消。"""