The summary path had grown a verbatim copy of turn_api_request's 3-line
surrogate/ASCII chokepoint — the same drift class this PR removes for the
hand-rolled kwargs builder. Move the two lines and the #50959 rationale into
`message_sanitization.sanitize_outbound_kwargs` and call it from both sites,
so the next sanitizer step added to the main loop cannot miss the summary.
Tighten two comments: "same kwargs builder" (cache_control redecoration is
not re-applied here) and a `_summary_text` note that is true for all three
summary branches, not just the chat one.
a19160bbf0