Function build_session_model_provider 

pub fn build_session_model_provider(
    config: &Config,
    model_provider_ref: &str,
    model_override: Option<&str>,
) -> Result<(Box<dyn ModelProvider>, String, String), Error>

Build a fresh ModelProvider box for a dotted <type>.<alias> reference, resolving the model from the override (when supplied) or the configured entry. Mirrors the model_provider-construction path in Agent::from_config so a live session switch produces the same wiring a fresh agent would. Returns the built box plus the resolved (model_provider_name, model_name) for attribution.