Cross-cutting helpers shared by ReqLLM.OpenTelemetry and ReqLLM.Telemetry.OpenTelemetry — option parsing, Langfuse cost-details merging, error rendering.

Both surfaces accept the same :content and :langfuse options and need to render error values the same way. This module is where that small but easy-to-drift logic lives.

content_mode/1 resolves the user-facing option to a canonical mode:

content_mode(content: :attributes)  #=> :attributes
content_mode(content: true)         #=> :attributes
content_mode(content: :event)       #=> :event
content_mode([])                    #=> :none

merge_langfuse/3 adds "langfuse.observation.cost_details" (a JSON-encoded input/output/reasoning/total breakdown) to an attribute map when langfuse: true is set and ReqLLM has cost data. If encoding fails it logs at debug and drops the attribute rather than raising — span emission keeps working.