Local code execution via builtins prefix allowlist bypass

Improper input validation in NVIDIA Megatron Bridge 0.4.0 and earlier allows local users to execute code through a crafted model configuration. The _contains_code_references function treats every builtins-prefixed target as safe, so instantiate resolves and invokes entries such as builtins.exec with attacker-controlled arguments. Exploitation requires a user to load the malicious configuration; execution occurs even when trust_remote_code is disabled and enables privilege escalation, data alteration, and information disclosure.