docs: Add code executor enhancements design document

Design doc covering three high-priority improvements to the ADK
code executor infrastructure:

1. Uniform timeout support across all executors (currently only
   GkeCodeExecutor has it)
2. Stateful Python execution in ContainerCodeExecutor (currently
   frozen to stateful=False)
3. Security hardening with a new LocalSandboxCodeExecutor and
   UnsafeLocalCodeExecutor restrictions

Also removes dead default parameter from _prepare_code(timeout=)
since timeout is now read directly from self._toolset._script_timeout
inside the shell branch.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: caohy1988