Previously only HTTP 404/503 and specific error strings triggered a fallback
to the main model when the summary model was unavailable. Timeout errors
(HTTP 408/429/502/504, or error strings containing 'timeout') entered a
short cooldown instead, leaving context to grow unbounded for the rest of
the session.
Add _is_timeout detection alongside _is_model_not_found so that transient
timeout errors on the summary model also trigger immediate fallback to the
main model, preventing compression failure from cascading.
Closes#15935
pander
6b88f46c54