[bot] OpenAI tracers do not set `braintrust.span_attributes` with `type: "llm"`

[AbhiPrasad]

Parent issue: #69

Summary

The OpenAI middleware tracers do not set the braintrust.span_attributes OTel attribute with type: "llm" on their spans, so OpenAI spans are not classified as LLM spans in the Braintrust UI.

Affected tracers

1. trace/contrib/openai/chatcompletions.go
2. trace/contrib/openai/responses.go

Expected behavior

Both tracers should set:

internal.SetJSONAttr(span, "braintrust.span_attributes", map[string]string{"type": "llm"})

Why this matters

Per https://www.braintrust.dev/docs/instrument/advanced-tracing, span_attributes.type = "llm" enables the LLM icon, LLM duration metrics, and the "Try prompt" feature in the Braintrust UI.

Acceptance criteria

• OpenAI Chat Completions tracer sets braintrust.span_attributes with type: "llm"
• OpenAI Responses tracer sets braintrust.span_attributes with type: "llm"
• Tests verify the attribute is actually present on emitted spans rather than hardcoding the expected value