Framework engines get event listeners and OpenTelemetry export

An Engine now delivers every event from every session it owns, including resumed and spawned ones, to the listeners registered on it. You no longer poll Session::events() per session. Each listener drains a bounded ordered queue, and shutdown flushes them within one deadline.

The everruns crate ships two listeners behind independent features, otel and braintrust:

let engine = Engine::builder()
    .observe(OpenTelemetry::from_env())
    .build();

Message content is not exported by default.

See Observability. PRs #3804, #3806.