fix(writer): 正文语言须与章节标题一致（模板日文则输出日文）

tests/test_phase5_writer_agent.py

@@ -54,6 +54,12 @@ def test_generate_chapter_success():
     assert content.blocks and content.blocks[0].type == "paragraph"
 
 
+def test_prompt_template_enforces_title_language():
+    # 语言约束：正文需与章节标题语言一致（模板为日文时输出日文）
+    assert "语言约束" in WRITER_PROMPT_TEMPLATE
+    assert "{{title}}" in WRITER_PROMPT_TEMPLATE
+
+
 def test_generate_chapter_retries():
     class Boom(FakeEngine):
         def chat_structured(self, *, session_id, prompt, variables, schema, retry_count=2):