Save and restore user span context on hibernation.

[mar-cf]

No description provided.

[ask-bonk]

The restore side is safe — when autogate is disabled, userSpanContext is kj::none, so .map(...) returns kj::none, meaning SubrequestMetadata.userSpanContext is kj::none and the previous behavior is preserved. The asymmetric gating (capture gated, restore not) is actually the correct pattern — it's simpler and safe.

This PR is clean. The changes are minimal, well-gated behind an autogate, and the types all align correctly. The test update correctly reflects the new parenting relationship.

LGTM

github run

[github-actions]

The generated output of @cloudflare/workers-types has been changed by this PR. If this is intentional, run just generate-types to update the snapshot. Alternatively, you can download the full generated types:

Full Type Diff