ResponseStream rejects with a plain object when the API emits an `error` event

[morgan-coded]

Streaming a Responses request that surfaces an API error frame rejects
finalResponse() with a plain object instead of an OpenAIError, so
instanceof-based error handling misses it.

Repro against the shipped class — no network, replaying a captured frame
through ResponseStream.fromReadableStream:

finalResponse REJECTED: isError=false isOpenAIError=false
raw= {"type":"error","code":"server_error","message":"boom","sequence_number":1}

Cause: error is a legitimate member of ResponseStreamEvent, so
#addEvent's default: arm re-emits it as an ordinary listener payload, and
EventStream._emit then rejects with the raw frame. The Assistants helper
handles the same class explicitly (AssistantStream.ts:381, "errors should be
processed earlier"), and the realtime emitter converts properly — Responses is
the only stream helper that doesn't.

Sending a PR alongside this: convert the frame in ResponseStream's error path,
mirroring the Assistants handling. The alternative is a transport-layer fix in
core/streaming.ts (which carries an unresolved TODO asking exactly this
question) — happy to move it there instead if you'd rather weigh the
Chat-Completions blast radius.