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/core/library_client.py

@@ -374,6 +374,10 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
         body = options.params or {}
         body |= options.json_data or {}
 
+        # Merge extra_json parameters (extra_body from SDK is converted to extra_json)
+        if hasattr(options, "extra_json") and options.extra_json:
+            body |= options.extra_json
+
         matched_func, path_params, route_path, webmethod = find_matching_route(options.method, path, self.route_impls)
         body |= path_params