agent: preserve otel child span events

[asim]

Summary

• Record model and tool run events before ending their OpenTelemetry spans so child spans keep the correlated timeline event instead of only the persisted run event.
• Assert model, tool, and stream spans include their corresponding run events.

Closes #3908
Closes #4025

Testing

• go build ./...
• go test ./agent
• go test ./... (fails in this environment because loopback gRPC tests are blocked by Envoy: "Loopback targets forbidden")
• golangci-lint run ./...