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

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.

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

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

docs/static/llama-stack-spec.yaml

@@ -1411,6 +1411,18 @@ paths:
               $ref: '#/components/schemas/CreateOpenaiResponseRequest'
         required: true
       deprecated: false
+      x-llama-stack-extra-body-params:
+        - name: shields
+          schema:
+            type: array
+            items:
+              oneOf:
+                - type: string
+                - $ref: '#/components/schemas/ResponseShieldSpec'
+          description: >-
+            List of shields to apply during response generation. Shields provide safety
+            and content moderation.
+          required: false
   /v1/responses/{response_id}:
     get:
       responses:
@@ -5739,6 +5751,18 @@ components:
       title: OpenAIResponseText
       description: >-
         Text response configuration for OpenAI responses.
+    ResponseShieldSpec:
+      type: object
+      properties:
+        type:
+          type: string
+          description: The type/identifier of the shield.
+      additionalProperties: false
+      required:
+        - type
+      title: ResponseShieldSpec
+      description: >-
+        Specification for a shield to apply during response generation.
     OpenAIResponseInputTool:
       oneOf:
         - $ref: '#/components/schemas/OpenAIResponseInputToolWebSearch'