openapi gen return type fix for streaming/non-streaming (#910)

# What does this PR do?

We need to change

```yaml
/v1/inference/chat-completion:
    post:
      responses:
        '200':
          description: >-
            If stream=False, returns a ChatCompletionResponse with the full completion.
            If stream=True, returns an SSE event stream of ChatCompletionResponseStreamChunk
          content:
            text/event-stream:
              schema:
                oneOf:
                  - $ref: '#/components/schemas/ChatCompletionResponse'
                  - $ref: '#/components/schemas/ChatCompletionResponseStreamChunk'
```

into

```yaml
/v1/inference/chat-completion:
    post:
      responses:
        '200':
          description: >-
            If stream=False, returns a ChatCompletionResponse with the full completion.
            If stream=True, returns an SSE event stream of ChatCompletionResponseStreamChunk
          content:
            text/event-stream:
              schema:
                $ref: '#/components/schemas/ChatCompletionResponseStreamChunk'
            application/json:
              schema:
                $ref: '#/components/schemas/ChatCompletionResponse'
```

## Test Plan

**Python**
- tested in SDK sync:
https://github.com/meta-llama/llama-stack-client-python/pull/108

**Node**
- tested w/
https://gist.github.com/yanxi0830/b782f4b91e21dcccdfef8898ce55157e (SDK
udpate follow up)


## Sources

Please link relevant resources if necessary.


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

docs/openapi_generator/pyopenapi/generator.py

@@ -177,20 +177,37 @@ class ContentBuilder:
     ) -> Dict[str, MediaType]:
         "Creates the content subtree for a request or response."
 
-        def has_iterator_type(t):
-            if typing.get_origin(t) is typing.Union:
-                return any(has_iterator_type(a) for a in typing.get_args(t))
+        def is_iterator_type(t):
+            return "StreamChunk" in str(t)
+
+        def get_media_type(t):
+            if is_generic_list(t):
+                return "application/jsonl"
+            elif is_iterator_type(t):
+                return "text/event-stream"
             else:
-                # TODO: needs a proper fix where we let all types correctly flow upwards
-                # and then test against AsyncIterator
-                return "StreamChunk" in str(t)
+                return "application/json"
+
+        if typing.get_origin(payload_type) is typing.Union:
+            media_types = []
+            item_types = []
+            for x in typing.get_args(payload_type):
+                media_types.append(get_media_type(x))
+                item_types.append(x)
+
+            if len(set(media_types)) == 1:
+                # all types have the same media type
+                return {media_types[0]: self.build_media_type(payload_type, examples)}
+            else:
+                # different types have different media types
+                return {
+                    media_type: self.build_media_type(item_type, examples)
+                    for media_type, item_type in zip(media_types, item_types)
+                }
 
         if is_generic_list(payload_type):
             media_type = "application/jsonl"
             item_type = unwrap_generic_list(payload_type)
-        elif has_iterator_type(payload_type):
-            item_type = payload_type
-            media_type = "text/event-stream"
         else:
             media_type = "application/json"
             item_type = payload_type