feat: add gen_ai.agent.name to chat spans

[c-ehrlich]

This PR adds the gen_ai.agent.name attribute to chat <model name> spans. chat spans carry the same name as the nearest parent invoke_agent span. This makes it easier for OTel backends to know which chat spans come from which agent without having to do relational checks.

An alternative solution would be to propagate gen_ai.agent.name to all child spans, but this is the smaller implementation.

[astrobot-houston]

Thanks for the contribution! We're closing this PR and moving the conversation to the existing thread: #384

We've moved to a model where bugs and feature proposals are discussed in issues/discussions before code review, so the community can help prioritize and shape the work. Your branch is linked from the new thread so the implementation isn't lost — please join us there to continue the conversation.

— astrobot 🤖