Harden Jinja template rendering with sandboxing

[tcoatswo]

Summary

Harden cloud-init Jinja template rendering by switching to a sandboxed Jinja environment.

Changes

• replace direct jinja2.Template rendering with SandboxedEnvironment
• handle SecurityError in the Jinja payload handler so unsafe templates are ignored cleanly
• add unit regressions covering unsafe attribute access in templater and handler paths

Verification

• reproduced the pre-fix behavior directly in Python: template expressions could access class, mro, and subclasses
• verified post-fix behavior directly in Python: unsafe attribute access is blocked, and handler rendering returns None with a warning
• I could not run the repo pytest suite on this host because the Python environment here does not have the pytest module installed