[deckhouse-cli] Fix extra images dropped on network errors

[Glitchy-Sheep]

Summary

During module pull, extra images are discovered by reading extra_images.json from each version image. A network error while reading it was treated the same as "this version has no extra images", so the images were dropped and the pull still reported success.

Problem

• findExtraImages swallowed every error from GetImage and from reading extra_images.json.
• A network error looked the same as "this version has no extra_images.json".
• img.Extract() made it worse: on a mid-layer read failure it returns a clean io.EOF, so even the network error itself was indistinguishable from an absent file.
• The dropped image never entered the download plan. The pull's retry and completeness check never saw it.
• The bundle ended up missing extra images, with no error shown to the user.

Fix

• Read extra_images.json by scanning the image layers directly instead of img.Extract(), so a truncated or failed layer read surfaces as a real error rather than a masked io.EOF.
• Read each version's extra_images.json through the shared retry.RunTask primitive (5 attempts, 10s apart) - the same retry the image-pull step already uses.
• A missing image or a missing extra_images.json is a clean skip.
• A transient error is retried. A persistent one is returned.
• findExtraImages now returns an error and pullExtraImages propagates it up.
• A persistent failure now fails the whole pull instead of writing an incomplete bundle.

Before / After

Before: a network error during extra images discovery dropped that version's extra images, and the pull still finished successfully.

After: the read is retried; if it keeps failing, the pull stops with a clear error instead of producing an incomplete bundle.

Tests

• TestFindExtraImages_Discovery - a version with extra_images.json yields its extra images; a version without it, or with a missing image, is skipped.
• TestFindExtraImages_RetriesTransientError - a transient GetImage error is retried and the extra images still land.
• TestFindExtraImages_FailsOnPersistentError - a persistent error returns an error instead of an empty result.
• TestExtractExtraImagesJSON_LayerReadErrorIsNotSkipped - a layer read that aborts mid-stream returns a real error, not a masked "not found".

Notes

• Scope is limited to the modules extra-images path. The same swallow-and-mask pattern exists in extractInternalDigestImages and extractVersionsFromReleaseChannels, whose pull step is best-effort, so they are left out on purpose.
• packages/packages.go findExtraImages is a near-duplicate of this code and has the same bug; a follow-up should apply the same fix there.