Function maybe_preprocess 

pub async fn maybe_preprocess(
    config: &Config,
    active_provider: &dyn LlmProvider,
    caption: &str,
    images: &[ImagePart],
) -> PreprocessOutcome

Decide whether and how to preprocess images before a main-provider turn.

Short-circuit order (each → Skipped, except the last):

1. images is empty.
2. The active provider’s model name passes the model_name_suggests_vision heuristic (it can handle the image natively).
3. config.vision_preprocessor_provider is None or Some("").
4. The configured key is missing from config.providers → Failed (this is a configuration mistake worth surfacing, not a silent skip).