[bot] Add embeddings instrumentation for OpenAI (openai-go)

[AbhiPrasad]

Summary

Split from #58.

trace/contrib/openai traces /v1/chat/completions and /v1/responses, but it does not route /v1/embeddings. Calls made through the official openai-go client via client.Embeddings.New() therefore fall through without emitting Braintrust spans.

What is missing

• Route /v1/embeddings in trace/contrib/openai/traceopenai.go
• Add an embeddings tracer that records model, input, usage/token metrics, and response metadata as a Braintrust llm span
• Add tests covering embeddings path matching and traced embeddings requests

Upstream surface

• github.com/openai/openai-go
• client.Embeddings.New()
• OpenAI Embeddings API: https://platform.openai.com/docs/api-reference/embeddings/create

Local repo files inspected

• trace/contrib/openai/traceopenai.go
• trace/internal/utils_test.go