feat(responses): add output_text delta events to responses (#2265)

This adds initial streaming support to the Responses API. 

This PR makes sure that the _first_ inference call made to chat
completions streams out.

There's more to be done:
 - tool call output tokens need to stream out when possible
- we need to loop through multiple rounds of inference and they all need
to stream out.

## Test Plan

Added a test. Executed as:

```
FIREWORKS_API_KEY=... \
  pytest -s -v 'tests/verifications/openai_api/test_responses.py' \
  --provider=stack:fireworks --model meta-llama/Llama-4-Scout-17B-16E-Instruct
```

Then, started a llama stack fireworks distro and tested against it like
this:

```
OPENAI_API_KEY=blah \
   pytest -s -v 'tests/verifications/openai_api/test_responses.py' \
   --base-url http://localhost:8321/v1/openai/v1 \
  --model meta-llama/Llama-4-Scout-17B-16E-Instruct 
```

tests/unit/providers/agents/meta_reference/test_openai_responses.py

@@ -232,9 +232,17 @@ async def test_create_openai_response_with_tool_call_type_none(openai_responses_
 
     # Check that we got the content from our mocked tool execution result
     chunks = [chunk async for chunk in result]
-    assert len(chunks) > 0
-    assert chunks[0].response.output[0].type == "function_call"
-    assert chunks[0].response.output[0].name == "get_weather"
+    assert len(chunks) == 2  # Should have response.created and response.completed
+
+    # Check response.created event (should have empty output)
+    assert chunks[0].type == "response.created"
+    assert len(chunks[0].response.output) == 0
+
+    # Check response.completed event (should have the tool call)
+    assert chunks[1].type == "response.completed"
+    assert len(chunks[1].response.output) == 1
+    assert chunks[1].response.output[0].type == "function_call"
+    assert chunks[1].response.output[0].name == "get_weather"
 
 
 @pytest.mark.asyncio