feat(livekit): add agents integration

[Abhijeet Prasad (AbhiPrasad)]

resolves https://linear.app/braintrustdata/issue/BT-5048

Add LiveKit Agents instrumentation, e2e VCR coverage, and nox/CI wiring for the livekit-agents matrix.

Also preserve OpenAI async streaming speech responses when tracing raw response calls.

import braintrust

braintrust.init_logger(project="your-project")
braintrust.auto_instrument()

from livekit.agents import Agent, AgentSession
from livekit.plugins import openai, silero

# create llm / stt / tts / vad / AgentSession after auto_instrument()

[Abhijeet Prasad (AbhiPrasad)]

included w/ pre-release pip install -i https://test.pypi.org/simple/ braintrust==0.19.0rc77

[Luca Forstner (lforst)]

wrt the screenshot in the pr descr: I think it is no bueno if we do two nested llm spans. That will double count tokens I believe. I would maybe make llm_requetst_run have type task.

[Abhijeet Prasad (AbhiPrasad)]

released another pre-release with latest changes pip install -i https://test.pypi.org/simple/ braintrust==0.20.0rc80