feat(responses)!: improve responses + conversations implementations (#3810)

This PR updates the Conversation item related types and improves a
couple critical parts of the implemenation:

- it creates a streaming output item for the final assistant message
output by
  the model. until now we only added content parts and included that
  message in the final response.

- rewrites the conversation update code completely to account for items
  other than messages (tool calls, outputs, etc.)

## Test Plan

Used the test script from
https://github.com/llamastack/llama-stack-client-python/pull/281 for
this

```
TEST_API_BASE_URL=http://localhost:8321/v1 \
  pytest tests/integration/test_agent_turn_step_events.py::test_client_side_function_tool -xvs
```

tests/integration/responses/fixtures/test_cases.py

@@ -212,7 +212,7 @@ multi_turn_image_test_cases = [
                 ),
                 (
                     "What country do you find this animal primarily in? What continent?",
-                    "peru",
+                    "south america",
                 ),
             ],
         ),