Add Robust SSE Buffering for OpenAI Stream Responses

[moulai]

• Add chunk buffering in helper.php::streamOpenAiApi() so partial SSE payloads are accumulated until a full event is available before decoding.
• Normalize line endings, parse event: / data: segments, and requeue incomplete JSON payloads to avoid fatal errors when chunks split mid-object.
• Buffer non-200 responses to capture the full error message before surfacing it to the client.