feat(api): add extra_body parameter support with shields example (#3670)

## Summary
Introduce `ExtraBodyField` annotation to enable parameters that arrive
via extra_body in client SDKs but are accessible server-side with full
typing.

These parameters are documented in OpenAPI specs under
**`x-llama-stack-extra-body-params`** but excluded from generated SDK
signatures.

Add `shields` parameter to `create_openai_response` as the first
implementation using this pattern.

## Test Plan
- added an integration test which checks that shields parameter passed
via extra_body reaches server implementation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>

llama_stack/providers/inline/agents/meta_reference/responses/openai_responses.py

@@ -208,10 +208,15 @@ class OpenAIResponsesImpl:
         tools: list[OpenAIResponseInputTool] | None = None,
         include: list[str] | None = None,
         max_infer_iters: int | None = 10,
+        shields: list | None = None,
     ):
         stream = bool(stream)
         text = OpenAIResponseText(format=OpenAIResponseTextFormat(type="text")) if text is None else text
 
+        # Shields parameter received via extra_body - not yet implemented
+        if shields is not None:
+            raise NotImplementedError("Shields parameter is not yet implemented in the meta-reference provider")
+
         stream_gen = self._create_streaming_response(
             input=input,
             model=model,