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
```

docs/static/deprecated-llama-stack-spec.yaml

@@ -6358,6 +6358,8 @@ components:
         - $ref: '#/components/schemas/OpenAIResponseInputFunctionToolCallOutput'
         - $ref: '#/components/schemas/OpenAIResponseMCPApprovalRequest'
         - $ref: '#/components/schemas/OpenAIResponseMCPApprovalResponse'
+        - $ref: '#/components/schemas/OpenAIResponseOutputMessageMCPCall'
+        - $ref: '#/components/schemas/OpenAIResponseOutputMessageMCPListTools'
         - $ref: '#/components/schemas/OpenAIResponseMessage'
     "OpenAIResponseInputFunctionToolCallOutput":
       type: object