Add _ensure_ssl_certs() that discovers CA certificate bundles before any
HTTP library is imported. Resolution order:
1. Python's ssl.get_default_verify_paths()
2. certifi (if installed)
3. Common distro/macOS paths
Only sets SSL_CERT_FILE if not already present in the environment.
Wrapped in a function (called immediately) to avoid polluting module
namespace.
Based on PR #1151 by sylvesterroos.
