[bot] Add embeddings instrumentation for Google GenAI (`google.golang.org/genai`)

[AbhiPrasad]

Summary

Split from #58.

trace/contrib/genai only routes generateContent and streamGenerateContent requests today. Calls to client.Models.EmbedContent() hit :embedContent endpoints and fall through without tracing for both Gemini API and Vertex AI style paths.

What is missing

• Route :embedContent / /embedContent requests in trace/contrib/genai/tracegenai.go
• Add an embeddings tracer that records model, inputs, usage/token metrics, and response metadata as a Braintrust llm span
• Add tests for Gemini and Vertex AI embeddings paths; trace/contrib/genai/tracegenai_test.go currently codifies /embedContent as a non-match

Upstream surface

• google.golang.org/genai
• client.Models.EmbedContent()
• Google Gemini Embeddings API: https://ai.google.dev/gemini-api/docs/embeddings

Local repo files inspected

• trace/contrib/genai/tracegenai.go
• trace/contrib/genai/tracegenai_test.go