OTel GenAI bridge for req_managed_agents (mirrors ReqLLM.OpenTelemetry).

Two surfaces:

• Pure mappers (attributes_for/2, delegating to Attributes) — the load-bearing capture surface a host (mimir-gateway) calls to normalize RMA events to gen_ai.*.
• Optional OTLP export (attach/1) — emits spans only when the OTel SDK is loaded; no-ops otherwise. No opentelemetry dependency is taken.

Scope

Both backends now run through the unified ReqManagedAgents.Session, which emits [:req_managed_agents, :session, :terminal | :tool_uses] (plus the streaming [:req_managed_agents, :stream | :tool, …] events) regardless of provider — so this bridge covers AgentCore and Claude Managed Agents alike. (The old per-driver [:req_managed_agents, :agent_core, …] events were retired with the driver collapse.)