Normalizes caller-facing inference options into the compact request_options map exposed on [:req_llm, :request, *] telemetry metadata.

The shape is OTel-attribute-friendly — keys map 1:1 to gen_ai.request.* attributes in ReqLLM.OpenTelemetry.Attributes — but the data lives in the telemetry context and is available to any consumer of the lifecycle events.

Keys whose value is nil are dropped so consumers never see blank-but-present fields.