feat(AI-GW): Guardrail plugins should support structured rejection responses and analytics

## Definition of done

ai-prompt-guard or ai-semantic-prompt-guard policies docs updated to include new responses and toggle config for stealth


## Information

https://konghq.atlassian.net/browse/AG-822


## Due date (optional)


## Size

- M (Medium) Example: writing a UI how to for a straightforward product, adding a plugin example

