test(server): stabilize async root span assertions

[pimlock]

Summary

Stabilizes async OpenTelemetry root-span assertions that can snapshot the in-memory exporter after child spans finish but before the root span is observable.

The reported linux-arm64 failure showed completed driver and store child spans without the expected reconcile.sandboxes root span: Actions failure.

A follow-up linux-amd64 run exposed the same symptom in the provider refresh test because its explicit current-span handle remained alive while the assertion polled: Actions failure.

This uses the same bounded polling pattern already used by the disconnected watch-span test added in #2582.

Related Issue

No issue required: this is an obvious localized CI test flake.

Changes

• Wait up to five seconds for the reconciliation root span, polling every 10 ms before taking the assertion snapshot.
• Audit the remaining async root-span tests and apply the same pattern to driver watch events and provider refresh ticks.
• Explicitly release the provider refresh current-span handle after recording attributes so the root span can close before exporter polling.
• Leave exporter tests that already explicitly drop their span handles unchanged.

Testing

• mise run pre-commit passes
• Focused async root-span tests pass
• mise run test passes, including all 1,228 server tests
• mise run ci completed lint and compile checks, but a supervisor-process test binary hung locally after its visible tests passed; stopped after five minutes
• E2E tests not applicable for telemetry test stabilization

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs not applicable for telemetry test stabilization