CommandCode returns errors as a type:"error" event inside an HTTP 200
NDJSON stream instead of a non-200 status, so the existing combo/account
fallback logic (keyed off response.status) never triggered and the error
text was streamed to the client as if it were content.
Peek the first NDJSON events before committing to a stream; on a
type:"error" event, abort and return a proper 4xx/5xx Response instead.
Normal streams are replayed losslessly (buffered prefix + rest of the
stream) through the existing translator, so the happy path is unchanged.
Add CommandCodeExecutor.parseError() so parseUpstreamError() can extract
a clean message/status from the synthesized error body.
