[bot] Add embeddings instrumentation for LangChainGo

[AbhiPrasad]

Summary

Split from #58.

trace/contrib/langchaingo traces LLM generation callbacks such as GenerateContent, but it does not emit spans for embeddings operations. LangChainGo embedding calls such as embedder.EmbedDocuments() and embedder.EmbedQuery() are currently invisible to Braintrust tracing.

What is missing

• Add embeddings callback handling in trace/contrib/langchaingo
• Emit Braintrust llm spans for embeddings requests, including model/provider metadata and relevant input/output details
• Add tests covering both document and query embedding flows

Upstream surface

• github.com/tmc/langchaingo
• embedder.EmbedDocuments()
• embedder.EmbedQuery()

Local repo files inspected

• trace/contrib/langchaingo/tracelangchaingo.go