Sync updates from stainless branch: main

[vladimirivic]

What does this PR do?

This is result of sync stainless script changes so that we can send correct header based on the stream arg:

• We modified spec to separate application/json from text/event-stream responses (openapi gen return type fix for streaming/non-streaming llama-stack#910)
• We modified stainless sync script to include text/event-stream when stream arg is True (https://github.com/fairinternal/llama-oss-operations/pull/68)