feat: sidekiq scheduled jobs improvements - `scheduled` operation and add links

[smoke]

When a Job is pushed via perform_in or friends, it goes through the following life-cycle:

1. First the Job is pushed with at attribute in Redis from Sidekiq::Client - this will now create <...> scheduled span (before it was <...> publish
2. Then the Job is handled from Sidekiq::Scheduled#enqueue and pushed without at attribute in Redis from Sidekiq::Client - this will keep creating <...> publish span
3. Then the Job is handled by the Worker as usual

Additionally when using :propagation_style = :link and :trace_poller_enqueue = true a handy set of links are added between the 3 actors above.

[thompson-tomo]

I think it is worthwhile considering the discussion in https://github.com/open-telemetry/semantic-conventions/issues/1688#issuecomment-3498476928 hence my comment https://github.com/open-telemetry/opentelemetry-ruby-contrib/pull/1717#discussion_r2483134544

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