Add buttons Field to OrchestrationResponse and TestOrchestrationResponse

[nuwangeek]

Priority: P1 — Foundation; all other tasks depend on this data shape
File: src/models/request_models.py
Type: Model change
OrchestrationResponse needs a new optional field:

buttons: Optional[List[Dict[str, Any]]] = Field(
    default=None,
    description="Optional list of choice buttons for MCQ step responses"
)

TestOrchestrationResponse needs the same field so the test endpoint does not silently drop button data.
Button dict shape matches the DMapper output:
{"title": "Button label shown to user", "payload": "#service, /POST/services/active/step_name"}
Work:

1. Add buttons field to OrchestrationResponse.
2. Add buttons field to TestOrchestrationResponse.
3. Confirm existing tests still pass (field is Optional, defaults to None, no breaking change).

[nuwangeek]

#367