The recovery block previously only retried (continue) when one of the
per-component sanitization checks (messages, tools, system prompt,
headers, credentials) found and stripped non-ASCII content. When the
non-ASCII lived only in api_messages' reasoning_content field (which
is built from messages['reasoning'] and not checked by the original
_sanitize_messages_non_ascii), all checks returned False and the
recovery fell through to the normal error path — burning a retry
attempt despite _force_ascii_payload being set.
Now the recovery always continues (retries) when _is_ascii_codec is
detected. The _force_ascii_payload flag guarantees the next iteration
runs _sanitize_structure_non_ascii(api_kwargs) on the full API payload,
catching any remaining non-ASCII regardless of where it lives.
Also adds test for the 'reasoning' field on canonical messages.
Fixes#6843
93b6f45224