fix(dialogs): F04 载入已保存定义后把原定义带入可见对话上下文

- 载入定义时以“用户消息+助手确认”形式把名称、含义、范围、缺失处理和表达式
  写入对话，后续“把刚才公式改成乘以 100”等指代可关联到已载入定义；
  生成前仍需显式确认，不静默沿用旧定义
- 选择空选项时同步清理当前定义与上下文，选择状态与预览保持一致
- 服务端单条消息长度上限 1000→2000 以容纳完整定义上下文，超长仍拒绝
- E2E 覆盖：载入后请求包含原定义、修改后表达式为 (收入−成本)×100、
  空选项清理；单测覆盖 2000/2001 边界

tests/test_formula_columns.py

@@ -200,6 +200,54 @@ def test_design_column_preserves_clarification_branch_and_full_conversation(form
     assert payload["input"][1:] == messages
 
 
+def test_design_column_accepts_loaded_definition_context(formula_data, settings):
+    ds, _ = formula_data
+    settings.AI_ENABLED = True
+    settings.OPENAI_API_KEY = "test-key"
+    settings.OPENAI_MODEL = "test-model"
+    context = "已载入已保存定义「收入成本差」；计算表达式：" + '{"operator":"subtract",' * 45
+    assert 1000 < len(context) <= 2000
+    messages = [
+        {"role": "user", "content": context},
+        {"role": "assistant", "content": "已载入该定义作为当前基准。"},
+        {"role": "user", "content": "把刚才公式改成乘以 100"},
+    ]
+    clarification = {
+        "status": "completed",
+        "output": [
+            {
+                "content": [
+                    {
+                        "type": "output_text",
+                        "text": '{"complete":false,"questions":["要乘在整个表达式上吗？"],"column":null}',
+                    }
+                ]
+            }
+        ],
+    }
+    with patch("finance.formula_views.httpx.Client") as http:
+        http.return_value.__enter__.return_value.post.return_value = type(
+            "Response",
+            (),
+            {"raise_for_status": lambda self: None, "json": lambda self: clarification},
+        )()
+        result = design_column(
+            messages,
+            ds,
+            {"current": ["2026-08"], "baseline": ["2025-08"], "filters": {}, "group": "department"},
+        )
+    assert result["complete"] is False
+    payload = http.return_value.__enter__.return_value.post.call_args.kwargs["json"]
+    assert payload["input"][1:] == messages
+
+    with pytest.raises(Exception, match="对话消息内容无效"):
+        design_column(
+            [{"role": "user", "content": "y" * 2001}],
+            ds,
+            {"current": ["2026-08"], "baseline": ["2025-08"], "filters": {}, "group": "department"},
+        )
+
+
 def test_design_column_returns_only_validated_formula_definition(formula_data, settings):
     ds, _ = formula_data
     settings.AI_ENABLED = True