fix: Hotfix streaming for prompts/agents

[jamesbaskerville]

Fixes regression to prompts.call streaming. Basically there was a refactor moving logic to raw_client files. In the process the streaming logic was changed and the method now does a yield _sse.data() where _sse.data is actually a string. This results in the enclosing method not actually yielding anything, so our streaming methods were very broken.

Simple hotfix here to remove the calling parens and yield the data string directly.

Confirmed working locally.

[jamesbaskerville]

fern bumped python generator and merged that instead