fix: streaming resource lock (#1879)

* fix: correct issue with handling lock during streaming

move locking for streaming into get_event_publisher call so it is locked and unlocked in the correct task for the streaming reponse

* fix: simplify exit stack management for create_chat_completion and create_completion

* fix: correct missing `async with` and format code

* fix: remove unnecessary explicit use of AsyncExitStack

fix: correct type hints for body_model

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Co-authored-by: Andrei <abetlen@gmail.com>