Prompt metadata now includes model config

Minor changes

• 3c9aa59 feat(ai): Prompts.get(..., with_metadata=True) results now include config, the JSON object of model parameters or agent configuration stored with the prompt version in PostHog prompt management (None when the version has none). Config is carried through the client-side cache and the stale-cache fallback. The hardcoded fallback string has no config, so use defensive access like (result.config or {}).get("temperature", 0). — Thanks @jurajmajerik!