Add cache instrumentation attributes to internal span as well

[jmorrell-cloudflare]

Following up on https://github.com/cloudflare/workerd/pull/5399#issuecomment-3572826534

My understanding is that adding the annotations to both the user and internal span currently fails because the internal span ownership is passed to the http client, so when we try to add result annotations everything fails.

This adds back the double-annotations and tries to unthread the parent span for cache http clients.