fix(api): 收紧 one-shot 响应字段契约

Align the one-shot book response payload with the canonical batch_direction request field so clients no longer depend on the legacy prompt key.
Also rename the targeted regression tests to match the tightened contract and assert that prompt is absent.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: papysans

backend/api/main.py

@@ -4615,7 +4615,7 @@ async def book_stage_callback(
     response_payload = {
         "project_id": project_id,
         "mode": req.mode.value,
-        "prompt": req.batch_direction.strip(),
+        "batch_direction": req.batch_direction.strip(),
         "continuation_mode": req.continuation_mode,
         "generated_chapters": len(created),
         "chapters": created,

backend/tests/test_api_smoke.py

@@ -1187,7 +1187,7 @@ def test_one_shot_book_stream_generation(self):
             self.assertIn("event: chapter_chunk", payload)
             self.assertIn("event: done", payload)
 
-    def test_one_shot_book_accepts_batch_direction_alias(self):
+    def test_one_shot_book_returns_canonical_batch_direction_field(self):
         project_id = self._create_project()
         batch_res = self.client.post(
             f"/api/projects/{project_id}/one-shot-book",
@@ -1199,10 +1199,12 @@ def test_one_shot_book_accepts_batch_direction_alias(self):
             },
         )
         self.assertEqual(batch_res.status_code, 200)
-        self.assertEqual(batch_res.json()["generated_chapters"], 1)
+        payload = batch_res.json()
+        self.assertEqual(payload["generated_chapters"], 1)
         self.assertEqual(
-            batch_res.json()["prompt"], "主角在雪夜被背叛后潜伏反击，最终揪出幕后主使。"
+            payload["batch_direction"], "主角在雪夜被背叛后潜伏反击，最终揪出幕后主使。"
         )
+        self.assertNotIn("prompt", payload)
 
     def test_consistency_p0_check_endpoint(self):
         project_id = self._create_project(taboo_constraints=["禁止词"])

backend/tests/test_v2_semantics.py

@@ -49,7 +49,7 @@ def test_project_creation_writes_project_type_and_synopsis_into_identity(self):
         self.assertIn("Story Synopsis", identity)
         self.assertIn("主角在霜城发现王室血脉秘密", identity)
 
-    def test_one_shot_book_accepts_batch_direction_alias(self):
+    def test_one_shot_book_returns_canonical_batch_direction_field(self):
         create_res = self.client.post(
             "/api/projects",
             json={
@@ -75,7 +75,8 @@ def test_one_shot_book_accepts_batch_direction_alias(self):
         self.assertEqual(batch_res.status_code, 200)
         payload = batch_res.json()
         self.assertEqual(payload["generated_chapters"], 1)
-        self.assertEqual(payload["prompt"], "主角在雪夜被背叛后潜伏反击，最终揪出幕后主使。")
+        self.assertEqual(payload["batch_direction"], "主角在雪夜被背叛后潜伏反击，最终揪出幕后主使。")
+        self.assertNotIn("prompt", payload)
 
     def test_one_shot_book_accepts_scope_field(self):
         create_res = self.client.post(