test: cover unique enqueue_at conflict schedules

[reneklacan]

Summary

• verify unique enqueue_at with Skip preserves the original schedule and payload
• verify unique enqueue_at with Replace uses the new schedule and payload
• assert sorted schedule ordering through the public Storage API

Testing

• cargo fmt --all -- --check
• cargo clippy --all-features --workspace --all-targets -- -D warnings
• cargo test -p oxana

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Note

Low Risk
Test-only additions with no production code changes.

Overview
Adds integration coverage for unique job conflicts on scheduled enqueue via enqueue_at.

Skip keeps the original scheduled_at and payload when a duplicate unique job is enqueued later. Replace updates both the schedule and payload, and schedule order is checked through list_scheduled.

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