Security ---- Unsandboxed code execution in PythonFunction

## Summary
A security issue exists in `PythonFunction.implementation` in
`langfun/core/coding/python/generation.py`.

The method calls `execution.run()` without explicit sandboxing,
which can allow arbitrary code execution in the host process when
processing untrusted input (LLM output, MCP server responses).

## Fix
A patch has been submitted in PR #692
## Disclosure
Reported to Google OSS VRP. Full details available upon request
to maintainers.

Would appreciate a review and merge of the linked PR.