Streaming responses now end with a data: [DONE] SSE event

Streaming responses for Chat Completions (/v1/chat/completions) and Completions (/v1/completions) now emit a standard data: [DONE] SSE event at the end of the stream, following the SSE standard (see the living specification).

The [DONE] marker is guaranteed to appear exactly once and is always the last event in the stream.