feat: expose serialization truncation constants in /config route (#7316)

* test: validate truncation logic and response structure

* feat: expose serialization truncation constants in /config route

* chore: tidy up redundant import from module

src/backend/tests/unit/api/v1/test_schemas.py

@@ -0,0 +1,82 @@
+import pytest
+from langflow.api.v1.schemas import VertexBuildResponse
+from langflow.serialization.constants import MAX_ITEMS_LENGTH
+
+expected_keys_vertex_build_response = {
+    "id",
+    "inactivated_vertices",
+    "next_vertices_ids",
+    "top_level_vertices",
+    "valid",
+    "params",
+    "data",
+    "timestamp",
+}
+expected_keys_data = {
+    "results",
+    "outputs",
+    "logs",
+    "message",
+    "artifacts",
+    "timedelta",
+    "duration",
+    "used_frozen_result",
+}
+expected_keys_outputs = {"message", "type"}
+
+
+def assert_vertex_response_structure(result):
+    assert set(result.keys()).issuperset(expected_keys_vertex_build_response)
+    assert set(result["data"].keys()).issuperset(expected_keys_data)
+    assert set(result["data"]["outputs"]["dataframe"].keys()).issuperset(expected_keys_outputs)
+
+
+def test_vertex_response_structure_without_truncate():
+    message = [{"key": 1, "value": 1}]
+    output_value = {"message": message, "type": "bar"}
+    data = {
+        "data": {"outputs": {"dataframe": output_value}, "type": "foo"},
+        "valid": True,
+    }
+
+    result = VertexBuildResponse(**data).model_dump()
+
+    assert_vertex_response_structure(result)
+    assert len(result["data"]["outputs"]["dataframe"]["message"]) == len(message)
+
+
+def test_vertex_response_structure_when_truncate_applies():
+    message = [{"key": i, "value": i} for i in range(MAX_ITEMS_LENGTH + 5000)]
+    output_value = {"message": message, "type": "bar"}
+    data = {
+        "data": {"outputs": {"dataframe": output_value}, "type": "foo"},
+        "valid": True,
+    }
+
+    result = VertexBuildResponse(**data).model_dump()
+
+    assert_vertex_response_structure(result)
+    assert len(result["data"]["outputs"]["dataframe"]["message"]) == MAX_ITEMS_LENGTH + 1
+
+
+@pytest.mark.parametrize(
+    ("size", "expected"),
+    [
+        (0, 0),
+        (42, 42),
+        (MAX_ITEMS_LENGTH, MAX_ITEMS_LENGTH),
+        (MAX_ITEMS_LENGTH + 1000, MAX_ITEMS_LENGTH + 1),
+        (MAX_ITEMS_LENGTH + 2000, MAX_ITEMS_LENGTH + 1),
+        (MAX_ITEMS_LENGTH + 3000, MAX_ITEMS_LENGTH + 1),
+    ],
+)
+def test_vertex_response_truncation_behavior(size, expected):
+    message = [{"key": i, "value": i} for i in range(size)]
+    output_value = {"message": message, "type": "bar"}
+    data = {
+        "data": {"outputs": {"dataframe": output_value}, "type": "foo"},
+        "valid": True,
+    }
+
+    result = VertexBuildResponse(**data).model_dump()
+    assert len(result["data"]["outputs"]["dataframe"]["message"]) == expected