Function execute_playground_inputs 

pub async fn execute_playground_inputs(
    inputs: PlaygroundInputs,
) -> Result<String, String>

In-process entry point for harn playground. Returns the captured stdout the CLI dispatcher would have printed, or a rendered error string.

This is the path tests use to exercise the playground driver without spawning the harn binary. Watch mode is not supported here — it has no terminal output contract worth asserting on.