add web search server-side tool spans to anthropic and openai instrumentation

[Andrew Kent (realark)]

new spec tests: braintrustdata/braintrust-spec#53

update anthropic and openai instrumentation to create server-side tool call spans

caveat: the instrumentation is on-par with python, but some info is simply not given by the llm vendor:

• web search latency is not reported (we just make the span start some time inside of the llm parent with a zero-length duration)
• cost is not reported (server tool use is and this can be used to update cost computation metrics in some cases)