[deckhouse-cli] fix: mirror: correct URL parsing for VEX images with port in registry address

[timmilesdw]

Description:

Problem

When running d8 mirror pull from a local registry with a port (e.g., localhost:443/deckhouse-etalon), the command fails with:

could not parse reference: localhost:443/deckhouse-etalon/localhost:443/deckhouse-etalon:sha256-xxx.att

The same command works correctly with remote registry.deckhouse.ru.

Root Cause

The FindVexImage function uses strings.SplitN(url, ":", 2) to split the URL into path and tag. For URLs containing a port, this splits at the first colon (the port separator) instead of the last colon (the tag separator):

Input:  localhost:443/deckhouse-etalon:sha256-xxx.att

SplitN(":", 2) result:
  imagePath = "localhost"                        ❌ (cut at port)
  tag = "443/deckhouse-etalon:sha256-xxx.att"    ❌

This incorrect parsing led to path duplication when constructing the final reference.

Solution

• Replace strings.SplitN with strings.LastIndex to find the last colon (before the tag)
• Remove unused segment manipulation code that was causing path duplication
• Add proper error handling for CheckImageExists — previously, errors other than ErrImageNotFound were silently ignored

Files Changed

• pkg/libmirror/images/digests.go
• pkg/libmirror/layouts/layouts.go

Notes

• Bug was introduced during registry client refactoring
• The new pull implementation (NEW_PULL=true in internal/mirror/platform/platform.go) already uses strings.LastIndex and doesn't have this issue