feat(inference): LLM 客户端全异步化（T8 架构审查整改）

- Issue9: client.py 由同步 httpx.Client 全异步化
  - LLMClient Protocol / HttpLLMClient.chat → async；httpx.AsyncClient + asyncio.sleep 退避
  - __enter__/__exit__ → __aenter__/__aexit__（async with 生命周期闭环）
- engine.py chat/chat_structured/_call 全部 async + await
- FakeLLMClient.chat → async；测试用 anyio pytest 插件转换（engine 32 + client 10 用例）
- 同步 inference-engine spec 与 milestone3 review 的 httpx 描述
- 全量 182 passed / 100.00%（987 stmts/252 br）

tests/test_inference_client.py

@@ -37,9 +37,13 @@ def test_client_implements_protocol():
     assert proto_params.issubset(impl_params)
 
 
-def test_chat_success():
+# ---------- T8: 全异步化（async 接口） ----------
+
+@pytest.mark.anyio
+async def test_chat_success_async():
+    """chat 为 async 接口，返回文本与用量（T8）。"""
     client = make_client(_ok_handler)
-    text, usage = client.chat(
+    text, usage = await client.chat(
         model="deepseek-chat",
         messages=[ChatMessage(role="user", content="hi")],
         temperature=0.2,
@@ -49,20 +53,16 @@ def test_chat_success():
     assert usage.input_tokens == 10 and usage.output_tokens == 5
 
 
-def test_chat_requires_api_key():
-    with pytest.raises(LLMNotConfiguredError):
-        HttpLLMClient(base_url="https://x", api_key="")
-
-
-def test_client_context_manager_closes_transport():
-    # with 块退出后底层 httpx.Client 应被关闭（释放连接）
-    with HttpLLMClient(
+@pytest.mark.anyio
+async def test_client_async_context_manager_closes_transport():
+    """async with 块退出后底层 httpx.AsyncClient 应被关闭（T8）。"""
+    async with HttpLLMClient(
         base_url="https://api.test.local",
         api_key="sk-test",
         transport=httpx.MockTransport(_ok_handler),
     ) as client:
         assert isinstance(client, HttpLLMClient)
-        text, _ = client.chat(
+        text, _ = await client.chat(
             model="deepseek-chat",
             messages=[ChatMessage(role="user", content="hi")],
             temperature=0.2,
@@ -72,18 +72,25 @@ def test_client_context_manager_closes_transport():
     assert client._client.is_closed is True
 
 
-def test_chat_timeout():
+def test_chat_requires_api_key():
+    with pytest.raises(LLMNotConfiguredError):
+        HttpLLMClient(base_url="https://x", api_key="")
+
+
+@pytest.mark.anyio
+async def test_chat_timeout():
     def slow(request):
         raise httpx.ReadTimeout("slow")
 
     with pytest.raises(LLMTimeoutError):
-        make_client(slow, retry_backoff=(0, 0)).chat(
+        await make_client(slow, retry_backoff=(0, 0)).chat(
             model="m", messages=[ChatMessage(role="user", content="x")],
             temperature=0.2, max_tokens=100,
         )
 
 
-def test_chat_5xx_retry_then_network_error():
+@pytest.mark.anyio
+async def test_chat_5xx_retry_then_network_error():
     calls = {"n": 0}
 
     def handler(request):
@@ -91,26 +98,28 @@ def test_chat_5xx_retry_then_network_error():
         return httpx.Response(500, text="boom")
 
     with pytest.raises(LLMNetworkError):
-        make_client(handler, retry_backoff=(0, 0)).chat(
+        await make_client(handler, retry_backoff=(0, 0)).chat(
             model="m", messages=[ChatMessage(role="user", content="x")],
             temperature=0.2, max_tokens=100,
         )
     assert calls["n"] == 3  # 初始 + 2 次退避重试（间隔 0/0.01）
 
 
-def test_chat_network_error_retry_then_network_error():
+@pytest.mark.anyio
+async def test_chat_network_error_retry_then_network_error():
     # 非超时的网络错误走 httpx.HTTPError 分支，重试耗尽后抛 LLMNetworkError
     def handler(request):
         raise httpx.ConnectError("connection refused")
 
     with pytest.raises(LLMNetworkError):
-        make_client(handler, retry_backoff=(0, 0)).chat(
+        await make_client(handler, retry_backoff=(0, 0)).chat(
             model="m", messages=[ChatMessage(role="user", content="x")],
             temperature=0.2, max_tokens=100,
         )
 
 
-def test_chat_4xx_no_retry():
+@pytest.mark.anyio
+async def test_chat_4xx_no_retry():
     calls = {"n": 0}
 
     def handler(request):
@@ -118,14 +127,15 @@ def test_chat_4xx_no_retry():
         return httpx.Response(429, text="rate limit")
 
     with pytest.raises(LLMNetworkError):
-        make_client(handler).chat(
+        await make_client(handler).chat(
             model="m", messages=[ChatMessage(role="user", content="x")],
             temperature=0.2, max_tokens=100,
         )
     assert calls["n"] == 1  # 4xx 不重试
 
 
-def test_chat_3xx_no_retry():
+@pytest.mark.anyio
+async def test_chat_3xx_no_retry():
     # 3xx（重定向，httpx 不自动跟随）不得被当作成功，应立即抛 LLMNetworkError
     calls = {"n": 0}
 
@@ -134,14 +144,15 @@ def test_chat_3xx_no_retry():
         return httpx.Response(302, headers={"Location": "https://elsewhere"})
 
     with pytest.raises(LLMNetworkError):
-        make_client(handler).chat(
+        await make_client(handler).chat(
             model="m", messages=[ChatMessage(role="user", content="x")],
             temperature=0.2, max_tokens=100,
         )
     assert calls["n"] == 1  # 3xx 不重试
 
 
-def test_chat_malformed_response_raises_llm_response_error():
+@pytest.mark.anyio
+async def test_chat_malformed_response_raises_llm_response_error():
     # 2xx 但响应体结构损坏（非 JSON / 缺 choices/message/content）→ LLMResponseError
     def not_json(request):
         return httpx.Response(200, text="not-json")
@@ -151,7 +162,7 @@ def test_chat_malformed_response_raises_llm_response_error():
 
     for handler in (not_json, missing_key):
         with pytest.raises(LLMResponseError):
-            make_client(handler).chat(
+            await make_client(handler).chat(
                 model="m", messages=[ChatMessage(role="user", content="x")],
                 temperature=0.2, max_tokens=100,
-            )
+            )