fix(api): escape the model name in the unroutable model error

Build the error body with sjson instead of formatting the requested model name into a JSON literal. The name is client supplied and is echoed back verbatim, so a quote in it produced a malformed body and a crafted name could append fields that overwrote the error code clients branch on, for example turning model_not_found into insufficient_quota.

Reported by the Codex reviewer on #4820.