Make Datadog conversation event logging opt-in

[simonrosenberg]

Problem

Conversation events from benchmark runs are always persisted to Datadog, which creates noisy logs and extra spend when persistence isn't required. We want this to be opt-in via an env toggle.

Linked work

• Benchmarks PR: Gate Datadog conversation event logging behind env flag #299
• Evaluation PR: https://github.com/OpenHands/evaluation/pull/144
• Software Agent SDK PR: Propagate Datadog conversation logging flag to evaluation dispatch software-agent-sdk#1703

Notes

Default behavior keeps logging disabled unless ENABLE_CONVERSATION_EVENT_LOGGING is set to a truthy value through the new workflow inputs. Enable it only when Datadog persistence is needed.