Function builtin_chain 

pub async fn builtin_chain(
    args: &[DslValue],
    ctx: &ReasoningBuiltinContext,
) -> Result<DslValue>

Execute the chain builtin: sequential execution where each step’s output feeds into the next step’s input.

Arguments:

• steps: list of maps, each with keys: system, prompt_template (optional) OR
• steps: list of strings (prompts, executed sequentially)

Returns the final step’s output as a string.