[deckhouse-cli] Fix misleading error message for registry path

[Glitchy-Sheep]

Summary

Pushing to a registry root (e.g. localhost:5000) failed with a raw go-containerregistry error about invalid characters. Users read it as "the port is not allowed". Now the CLI says what is actually missing: the repository path.

Problem

• d8 mirror push distr/ localhost:5000 --insecure fails with:
  Validate registry address: repository can only contain the characters 'abcdefghijklmnopqrstuvwxyz0123456789_-./': localhost:5000
• The real cause: the address must include a repository path (localhost:5000/deckhouse). Pushing to a registry root is not supported.
• The message points at characters, and the only suspicious character in the input is the : before the port.
• Root cause: without a / in the address, name.NewRepository validates the whole host:port string as a repository name, where : is illegal. The friendlier host and path checks below never run.

Fix

• Check for a missing repository path before calling name.NewRepository.
• Applied to both inputs: mirror push <registry> and mirror pull --source.
• The new error names the argument, shows the expected format, and builds an example from the user's own input.
• Remaining parse errors now name the argument instead of the generic Validate registry address: wrapper.
• Docs: format example added to the mirror README and the --source flag help.
• Error-path only: every address that was valid before stays valid (pinned by tests).

Before / After

Before:

After:

Tests

• TestParseAndValidateRegistryURLArg (push): new cases for localhost:5000, http:// scheme stripping, trailing slash, and the full new message text.
• Same test: backward-compat pins for 192.168.1.1:5000/repo, myregistry/repo, localhost:5000//repo - they stay accepted.
• TestValidationValidateSourceRegistry (pull): new cases for localhost:5000 and trailing slash; the existing "no path" case now asserts the new message.