M2b：删除旧 chat 体系——线性会话/叩心/情绪/纪念日模型与页面退役（迁移 003 双向验证），runs 去会话外键，测试全量修剪

backend/tests/test_errors.py

@@ -34,23 +34,8 @@ async def test_error_contracts(client: AsyncClient) -> None:
     assert missing_run.status_code == 404
     assert_error(missing_run, "not_found")
 
-    validation = await client.post("/api/v1/chat/sessions", json={"title": "x" * 300})
+    validation = await client.post("/api/v1/runs", json={"prompt": ""})
     assert validation.status_code == 422
     assert_error(validation, "validation_failed")
 
-    created = await client.post("/api/v1/chat/sessions", json={"title": "错误"})
-    session_id = created.json()["session_id"]
-    unsupported = await client.post(
-        f"/api/v1/chat/sessions/{session_id}/messages",
-        json={"content": "你好", "modality": "image"},
-    )
-    assert unsupported.status_code == 415
-    assert_error(unsupported, "unsupported_media_type")
-
-    too_large = await client.post(
-        f"/api/v1/chat/sessions/{session_id}/messages",
-        json={"content": "x" * 40000, "modality": "text"},
-    )
-    # 当前消息内容无长度上限限制，接受 200；若后续加回来再改为 assert 413
-    if too_large.status_code == 413:
-        assert_error(too_large, "payload_too_large")
+    # 旧 chat 会话错误契约随线性会话模型退役；415/413 契约由镜树/媒体域随需要重建