test(sync): add e2e coverage for asynchronous on-demand mirror caching

[tehlers320]

Description

Add an end-to-end test for asynchronous on-demand mirror caching. The test should configure zot with onDemand: true and asyncOnDemand: true, request an uncached manifest through a containerd-style mirror setup, verify the first request falls back to the upstream registry, and verify a later request is served from zot after the background cache fill completes.

Acceptance criteria

• The first uncached manifest request receives a cache-miss response from zot.
• Zot fills the image cache in the background.
• A later request for the same reference is served from zot.
• Concurrent misses trigger one effective image sync.

This issue tracks the e2e automation requested by the pull request template.