fix: validate URL schemes and single-label hosts

[puneetdixit200]

Summary

• reject non-HTTP(S) URLs in the shared URL validator with URL scheme must be http or https
• allow RFC 1123-style single-label hosts such as http://harbor and https://registry:8443
• add regression coverage for unsupported schemes and single-label host edge cases

Fixes #937.
Fixes #879.

Verification

• go test ./pkg/utils
• go test ./...
• go build -o ./bin/harbor-cli cmd/harbor/main.go
• ./bin/harbor-cli --help
• git diff --check

AI disclosure

This contribution was prepared with AI assistance and reviewed before submission. I verified the changed validator logic and tests locally.