Add support for o3/Claude with web search

OpenAI and Anthropic both offer reasoning models with built-in web search capabilities. While I'm not necessarily a fan of the foundation models building in agent tools in general, I do think web search is a particular case where letting the foundation model use it can offer a lot of benefits. So we should add library support for using these models WITH web search enabled. I'm not sure on the best DX, but maybe we could add `reasoning_tools` as a property to the agent: ``` agent = Agent(..., tools=X, reasoning_tools=["websearch"]) ``` Here is the LiteLLM page describing their support: https://docs.litellm.ai/docs/completion/web_search So we could just copy that. One thing that isn't clear to me is whether this is an `agent-level` or `completion-level` feature. If we do it at completion level then we need to add the option to the `Prompt` event.. but I'm not sure how the caller knows to set that flag? My hunch is to make it an Agent-level setting, and to hope that the model is smart enough to only use web search when it needs it!