Router shipped a minimal /v1/chat/completions compatibility surface
(translated onto Responses) after this PR was written, so the
'does not exist and 404s' wording is stale. Responses remains the
native wire — per-model reasoning-effort validation, reasoning
summaries, and prompt caching live there — so the api.router.com
host mandate is unchanged; only the comments and docs are updated.
ca3060b70f