Function merged 

pub fn merged(
    record: &ModelRecord,
    capability: &Capability,
    payload: JsonValue,
    fallback_prompt: Option<&str>,
    session_prompt: Option<&str>,
    appended_block: Option<&str>,
) -> JsonValue

Merge the record’s configuration into a request payload.

For a chat capability, a system prompt (session override, else the record’s own, else fallback_prompt) is seeded into the messages array, and any appended_block is folded in. Saved parameter values fill in keys the payload does not already set. Non-object, non-null payloads pass through untouched.