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/verifications/openai_api/conftest.py

@@ -10,17 +10,17 @@ from tests.verifications.openai_api.fixtures.fixtures import _load_all_verificat
 def pytest_generate_tests(metafunc):
     """Dynamically parametrize tests based on the selected provider and config."""
     if "model" in metafunc.fixturenames:
+        model = metafunc.config.getoption("model")
+        if model:
+            metafunc.parametrize("model", [model])
+            return
+
         provider = metafunc.config.getoption("provider")
         if not provider:
             print("Warning: --provider not specified. Skipping model parametrization.")
             metafunc.parametrize("model", [])
             return
 
-        model = metafunc.config.getoption("model")
-        if model:
-            metafunc.parametrize("model", [model])
-            return
-
         try:
             config_data = _load_all_verification_configs()
         except (OSError, FileNotFoundError) as e: