feat(ext): add VettoCommandLineCodeExecutor for unprivileged sandboxed execution (#8298)

[Shleder (shleder)]

Why are these changes needed?

In autogen-ext, code execution currently defaults to LocalCommandLineCodeExecutor when Docker is not installed or unavailable. This executes untrusted code directly on the host machine without isolation boundaries, exposing user files, environment variables, and network sockets.

This PR adds VettoCommandLineCodeExecutor to autogen-ext as an unprivileged, kernel-level sandbox executor:

• Lightweight sandbox runtime (<4ms startup, 0MB daemon RAM) utilizing Landlock LSM (Linux), Seatbelt (macOS), and LPAC (Windows).
• Configurable network isolation (off, allowlist, host).
• Memory limits enforced via cgroups v2.
• Clean process-group termination and fail-closed timeout handling (exit code 124/125).
• Integrated into create_default_code_executor to prefer an isolated sandbox over un-sandboxed local execution when Docker is absent.

Related issue number

Closes #8298

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/.
• I've added tests corresponding to the changes introduced in this PR (test_vetto_commandline_code_executor.py).
• I've made sure all auto checks have passed.

[Shleder (shleder)]

@microsoft-github-policy-service agree