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>

tests/integration/responses/test_extra_body_shields.py

@@ -0,0 +1,33 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+"""
+Test for extra_body parameter support with shields example.
+
+This test demonstrates that parameters marked with ExtraBodyField annotation
+can be passed via extra_body in the client SDK and are received by the
+server-side implementation.
+"""
+
+import pytest
+from llama_stack_client import APIStatusError
+
+
+def test_shields_via_extra_body(compat_client, text_model_id):
+    """Test that shields parameter is received by the server and raises NotImplementedError."""
+
+    # Test with shields as list of strings (shield IDs)
+    with pytest.raises((APIStatusError, NotImplementedError)) as exc_info:
+        compat_client.responses.create(
+            model=text_model_id,
+            input="What is the capital of France?",
+            stream=False,
+            extra_body={"shields": ["test-shield-1", "test-shield-2"]},
+        )
+
+    # Verify the error message indicates shields are not implemented
+    error_message = str(exc_info.value)
+    assert "not yet implemented" in error_message.lower() or "not implemented" in error_message.lower()