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/apis/agents/agents.py

@@ -28,7 +28,7 @@ from llama_stack.apis.inference import (
 from llama_stack.apis.safety import SafetyViolation
 from llama_stack.apis.tools import ToolDef
 from llama_stack.apis.version import LLAMA_STACK_API_V1, LLAMA_STACK_API_V1ALPHA
-from llama_stack.schema_utils import json_schema_type, register_schema, webmethod
+from llama_stack.schema_utils import ExtraBodyField, json_schema_type, register_schema, webmethod
 
 from .openai_responses import (
     ListOpenAIResponseInputItem,
@@ -42,6 +42,20 @@ from .openai_responses import (
 )
 
 
+@json_schema_type
+class ResponseShieldSpec(BaseModel):
+    """Specification for a shield to apply during response generation.
+
+    :param type: The type/identifier of the shield.
+    """
+
+    type: str
+    # TODO: more fields to be added for shield configuration
+
+
+ResponseShield = str | ResponseShieldSpec
+
+
 class Attachment(BaseModel):
     """An attachment to an agent turn.
 
@@ -805,6 +819,12 @@ class Agents(Protocol):
         tools: list[OpenAIResponseInputTool] | None = None,
         include: list[str] | None = None,
         max_infer_iters: int | None = 10,  # this is an extension to the OpenAI API
+        shields: Annotated[
+            list[ResponseShield] | None,
+            ExtraBodyField(
+                "List of shields to apply during response generation. Shields provide safety and content moderation."
+            ),
+        ] = None,
     ) -> OpenAIResponseObject | AsyncIterator[OpenAIResponseObjectStream]:
         """Create a new OpenAI response.
 
@@ -812,6 +832,7 @@ class Agents(Protocol):
         :param model: The underlying LLM used for completions.
         :param previous_response_id: (Optional) if specified, the new response will be a continuation of the previous response. This can be used to easily fork-off new responses from existing responses.
         :param include: (Optional) Additional fields to include in the response.
+        :param shields: (Optional) List of shields to apply during response generation. Can be shield IDs (strings) or shield specifications.
         :returns: An OpenAIResponseObject.
         """
         ...