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/fixtures/test_cases/responses.yaml

@@ -77,11 +77,12 @@ test_response_image:
           image_url: "https://upload.wikimedia.org/wikipedia/commons/f/f7/Llamas%2C_Vernagt-Stausee%2C_Italy.jpg"
       output: "llama"
 
+# the models are really poor at tool calling after seeing images :/
 test_response_multi_turn_image:
   test_name: test_response_multi_turn_image
   test_params:
     case:
-    - case_id: "llama_image_search"
+    - case_id: "llama_image_understanding"
       turns:
       - input:
         - role: user
@@ -91,7 +92,5 @@ test_response_multi_turn_image:
           - type: input_image
             image_url: "https://upload.wikimedia.org/wikipedia/commons/f/f7/Llamas%2C_Vernagt-Stausee%2C_Italy.jpg"
         output: "llama"
-      - input: "Search the web using the search tool for the animal from the previous response. Your search query should be a single phrase that includes the animal's name and the words 'maverick', 'scout' and 'llm'"
-        tools:
-        - type: web_search
-        output: "model"
+      - input: "What country do you find this animal primarily in? What continent?"
+        output: "peru"