Expose a transport-level `cancel`/`close` handle on async streaming responses (so they can be cancelled from another task)

[adtyavrdhn]

While working on pydantic/pydantic-ai#1524 I hit: generate_content_stream only exposes the generator, so cancelling it from another task while we're iterating raises RuntimeError: aclose(): asynchronous generator is already running.

Describe the solution you'd like
A transport handler we could close with or cancel with would be great

Describe alternatives you've considered
There are some things we could do to maybe mimic this but given that most SDKs have this it would be nice if genai did too

Additional context
I have just added a small warning for now.
https://pydantic.dev/docs/ai/models/google/#streaming-cancellation

[Venkaiahbabuneelam]

Hi @adtyavrdhn, Thanks for reaching out us!

Thank you for your feedback. We appreciate you taking the time to share your thoughts with us, and we'll be filing a feature request.

Thanks