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.html

@@ -7540,6 +7540,9 @@
                     {
                         "$ref": "#/components/schemas/OpenAIResponseObjectStreamResponseCreated"
                     },
+                    {
+                        "$ref": "#/components/schemas/OpenAIResponseObjectStreamResponseOutputTextDelta"
+                    },
                     {
                         "$ref": "#/components/schemas/OpenAIResponseObjectStreamResponseCompleted"
                     }
@@ -7548,6 +7551,7 @@
                     "propertyName": "type",
                     "mapping": {
                         "response.created": "#/components/schemas/OpenAIResponseObjectStreamResponseCreated",
+                        "response.output_text.delta": "#/components/schemas/OpenAIResponseObjectStreamResponseOutputTextDelta",
                         "response.completed": "#/components/schemas/OpenAIResponseObjectStreamResponseCompleted"
                     }
                 }
@@ -7590,6 +7594,41 @@
                 ],
                 "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": {