GHSA-6vh2-h83c-9294: PraisonAI: Python Sandbox Escape via str Subclass startswith() Override in execute_code

The `execute_code()` function in PraisonAI uses a sandbox to restrict what Python code can do, but attackers can bypass all three security layers by creating a custom `str` subclass (a modified version of the string type) with an overridden `startswith()` method, allowing them to run arbitrary OS commands on the host system. This is especially dangerous because many deployments auto-approve code execution without human review, so an attacker could trigger the vulnerability silently through indirect prompt injection (sneaking malicious instructions into the AI's input).