telemetry(core): trace terminal event delivery

[apanasenko-oai]

Summary

Make the terminal exec event path attributable after command completion.

• Trace primary and legacy event construction, rollout append, protocol recording, downstream delivery, and the client channel barrier.
• Trace parent notification and realtime cleanup only for exec-end events.
• Preserve event payloads and delivery ordering.

This is the terminal event-delivery tracing slice split out of #30632.

Validation

• Four focused exec begin, exec end, fallback output, and session delivery tests passed.
• just fmt passed, with an unrelated justfile rewrite discarded.