fix(codex): don't let Responses-API passthrough model bypass account mapping

convertToCodexFormat unconditionally overwrote the account-mapped model
with the raw client-requested model whenever Responses-API passthrough
data was present, silently defeating per-account model_mappings and
family-tier defaults for all Codex Responses traffic. Now the raw
passthrough model is only used as a fallback when no explicit account
mapping applies. Also fixes reasoning-effort resolution to check the
model actually sent instead of the discarded mapped one.