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>

docs/openapi_generator/pyopenapi/specification.py

@@ -106,6 +106,15 @@ class Parameter:
     example: Optional[Any] = None
 
 
+@dataclass
+class ExtraBodyParameter:
+    """Represents a parameter that arrives via extra_body in the request."""
+    name: str
+    schema: SchemaOrRef
+    description: Optional[str] = None
+    required: Optional[bool] = None
+
+
 @dataclass
 class Operation:
     responses: Dict[str, Union[Response, ResponseRef]]
@@ -118,6 +127,7 @@ class Operation:
     callbacks: Optional[Dict[str, "Callback"]] = None
     security: Optional[List["SecurityRequirement"]] = None
     deprecated: Optional[bool] = None
+    extraBodyParameters: Optional[List[ExtraBodyParameter]] = None
 
 
 @dataclass