fix(validators): allow nested GitLab repository paths

[go165]

Summary

• Allow GitLab repository URLs with nested groups/subgroups in repository validation
• Keep GitHub validation limited to owner/repo paths
• Add direct URL validation coverage and a full server.json semantic validation regression case

Fixes #1359.

Validation

• git diff --check
• Equivalent regex smoke test with local Go 1.22.2 outside this module
• go test ./internal/validators could not run in this environment because this module requires Go >= 1.26 and automatic toolchain download failed with go: download go1.26 for linux/amd64: toolchain not available; GOTOOLCHAIN=local go test ./internal/validators also fails because local Go is 1.22.2.