Own the surrogate-crash class at three chokepoints instead of leaf sites:
- finalize_turn scrubs final_response once where model text leaves the
conversation loop — covers oneshot stdout (#80366), NIM/any-provider
responses (#19819), and every delivery consumer of the turn result.
- _sanitize_gateway_final_response scrubs at the gateway chat-surface
boundary — Telegram utf16_len (#55309) and Signal formatting (#55143)
can no longer see a lone surrogate; raw-text surfaces keep passthrough.
- run_conversation walks the fully-built api_kwargs with
_sanitize_structure_surrogates so tool descriptions (session_search,
#50959) and every other request-body leaf are JSON-encodable before
any provider sees them.
Regression tests pin all three chokepoints plus helper semantics.
Cherry-picked alongside #79240 (TheophilusChinomona) and #80374
(rainbowgore) whose commits precede this one with authorship preserved.
Teknium