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 
```

docs/_static/llama-stack-spec.yaml

@@ -5294,11 +5294,13 @@ components:
     OpenAIResponseObjectStream:
       oneOf:
         - $ref: '#/components/schemas/OpenAIResponseObjectStreamResponseCreated'
+        - $ref: '#/components/schemas/OpenAIResponseObjectStreamResponseOutputTextDelta'
         - $ref: '#/components/schemas/OpenAIResponseObjectStreamResponseCompleted'
       discriminator:
         propertyName: type
         mapping:
           response.created: '#/components/schemas/OpenAIResponseObjectStreamResponseCreated'
+          response.output_text.delta: '#/components/schemas/OpenAIResponseObjectStreamResponseOutputTextDelta'
           response.completed: '#/components/schemas/OpenAIResponseObjectStreamResponseCompleted'
     "OpenAIResponseObjectStreamResponseCompleted":
       type: object
@@ -5330,6 +5332,33 @@ components:
         - type
       title: >-
         OpenAIResponseObjectStreamResponseCreated
+    "OpenAIResponseObjectStreamResponseOutputTextDelta":
+      type: object
+      properties:
+        content_index:
+          type: integer
+        delta:
+          type: string
+        item_id:
+          type: string
+        output_index:
+          type: integer
+        sequence_number:
+          type: integer
+        type:
+          type: string
+          const: response.output_text.delta
+          default: response.output_text.delta
+      additionalProperties: false
+      required:
+        - content_index
+        - delta
+        - item_id
+        - output_index
+        - sequence_number
+        - type
+      title: >-
+        OpenAIResponseObjectStreamResponseOutputTextDelta
     CreateUploadSessionRequest:
       type: object
       properties: