feat: Improve OpenAPIServiceConnector service response serialization (#6772)

* Better service response json -> str serialization

* Add unit test

haystack/components/connectors/openapi_service.py

@@ -63,7 +63,12 @@ class OpenAPIServiceConnector:
         response_messages = []
         for method_invocation_descriptor in function_invocation_payloads:
             service_response = self._invoke_method(openapi_service, method_invocation_descriptor)
-            response_messages.append(ChatMessage.from_user(str(service_response)))
+            # openapi3 parses the JSON service response into a model object, which is not our focus at the moment.
+            # Instead, we require direct access to the raw JSON data of the response, rather than the model objects
+            # provided by the openapi3 library. This approach helps us avoid issues related to (de)serialization.
+            # By accessing the raw JSON response through `service_response._raw_data`, we can serialize this data
+            # into a string. Finally, we use this string to create a ChatMessage object.
+            response_messages.append(ChatMessage.from_user(json.dumps(service_response._raw_data)))
 
         return {"service_response": response_messages}
 

test/components/connectors/test_openapi_service.py

@@ -2,6 +2,7 @@ import json
 import pytest
 from unittest.mock import MagicMock, Mock
 from openapi3 import OpenAPI
+from openapi3.schemas import Model
 from haystack.components.connectors import OpenAPIServiceConnector
 from haystack.dataclasses import ChatMessage
 
@@ -79,3 +80,11 @@ class TestOpenAPIServiceConnector:
         method_invocation_descriptor = {"name": "invalid_method", "arguments": {}}
         with pytest.raises(RuntimeError):
             connector._invoke_method(openapi_service_mock, method_invocation_descriptor)
+
+    def test_for_internal_raw_data_field(self):
+        # see https://github.com/deepset-ai/haystack/pull/6772 for details
+        model = Model(data={}, schema={})
+        assert hasattr(model, "_raw_data"), (
+            "openapi3 changed. Model should have a _raw_data field, we rely on it in OpenAPIServiceConnector"
+            " to get the raw data from the service response"
+        )