[alexcastgonz (pcbeingused333)]

Messages streaming can return a partial answer as a successful completion when the provider sends an SSE error event, such as overloaded_error, after a text delta.

The Messages stream event union omitted error events, so the decoder silently returned an empty piece. Add the error event type and return its message as StreamPiece.error. The existing stream consumer throws that error, preventing the partial output from being treated as a completed response.

Testing and validation

Add parameterized regression coverage for provider errors both before any text and after a partial text delta. The tests assert that the decoder returns an Error with the provider message and no text or usage on the error piece.

Both new cases fail on main (expected undefined to be an instance of Error) and pass with this change; the rest of the MessagesWireApiProvider suite still passes. No paid API calls are required.

This change was prepared with AI assistance (Codex).

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