Add URL validation helper to prevent URL injection attacks in API calls.
- Added validate_url() helper in lib/env.sh to validate URL format
- Added validation to forge_api() to prevent URL injection
- Added validation to woodpecker_api() to prevent URL injection
- Added validation to ci-debug.sh api() function
- All URLs are already properly quoted with "${VAR}/..." patterns
- This adds defense-in-depth by validating URL variables before use