Function log_payload_shape 

fn log_payload_shape(
    body: &Value,
    messages: &[ChatMessage],
    before_count: usize,
)

Diagnostics for tracking down oversized payloads: the byte cost of every non-message top-level field, and the content size of every message.

Gated on the log level as a whole because both loops serialise their way to a size — work the debug! macro would otherwise discard after paying for it on every strict-turn request.