When the empty-response ladder is fully exhausted (thinking-prefill
continuation, empty-content retries, provider fallback) and the model
produced structured reasoning but never any visible text, deliver a
clearly labeled excerpt of that reasoning instead of a bare '(empty)' —
the reasoning frequently contains the actual answer.
Delivery-only by design: raw chain-of-thought is never promoted to a
normal answer earlier in the ladder (prefill continuation still gets
first crack, retries and fallback still run), transcript persistence
semantics are untouched (the '(empty)' sentinel scaffolding keeps its
replay-safety behavior), and a truly empty exhaustion still returns the
existing terminal.
Idea credit: PR #48795 (@ligl0325) proposed falling back to
reasoning_content on empty content; this lands the safe kernel of that
idea at the one point in the ladder where it is strictly an improvement.
