fix: allow nested GitLab repository groups

[go165]

Summary

• Allow GitLab repository URLs with nested group/subgroup paths.
• Keep each GitLab path segment non-empty so malformed URLs with empty segments are still rejected.
• Add validator coverage for a GitLab repository under nested subgroups.

Fixes #1359.

Verification

• gofmt -w internal/validators/utils.go internal/validators/validators_test.go
• git diff --check
• Attempted go test ./internal/validators, but local verification is blocked by the Go 1.26 toolchain requirement: go: download go1.26 for linux/amd64: toolchain not available