fix(writer): 透传底层 LLM 错误详情（401 等）便于门禁定位

tests/test_phase5_writer_agent.py

@@ -88,3 +88,26 @@ def test_generate_chapter_with_async_engine():
     content = agent.generate_chapter(_ctx("db_design", "DB 設計"))
     assert content.chapter_id == "db_design"
     assert content.blocks and content.blocks[0].type == "paragraph"
+
+
+def test_generate_chapter_propagates_engine_error_detail():
+    class FailingEngine:
+        def chat_structured(self, *, session_id, prompt, variables, schema, retry_count=2):
+            return SimpleNamespace(
+                data={},
+                status="failed",
+                error="LLM HTTP 401: invalid key",
+                error_code="LLM_NETWORK_ERROR",
+            )
+
+    agent = WriterAgent(
+        session_id="s",
+        engine=FailingEngine(),
+        prompt_registry=FakePromptRegistry(),
+        state=WriterState(["db_design"]),
+        max_retries=1,
+    )
+    with pytest.raises(WriterGenerationError) as exc:
+        agent.generate_chapter(_ctx("db_design", "DB 設計"))
+    assert "LLM HTTP 401" in str(exc.value)
+    assert "LLM_NETWORK_ERROR" in str(exc.value)