fix: block os.system via the nt module on Windows

[nolanchic]

What does this PR do?

Closes #2232.

check_safer_result blocks dangerous functions by matching result.__module__ + "." + result.__name__ against DANGEROUS_FUNCTIONS. On Windows, os.system is implemented in the nt module, so os.system.__module__ is "nt" — not "os". Since DANGEROUS_FUNCTIONS listed os.system and posix.system but not nt.system, the check silently let os.system through on Windows when a user authorized os via additional_authorized_imports=["os"].

The default sandbox blocks os entirely, so this only affects users who explicitly authorize it — but for them it is a sandbox escape on Windows.

How

Add nt.system to DANGEROUS_FUNCTIONS, mirroring the existing posix.system entry that covers Linux/macOS.

Test

• The existing parametrized test_vulnerability_for_all_dangerous_functions now covers nt.system on Windows. It uses pytest.importorskip, so nt.system is skipped on Linux/macOS — exactly symmetric to how posix.system is skipped on Windows.
• Added test_check_safer_result_blocks_nt_system, which is platform-independent: it passes check_safer_result a function with __module__ == "nt" / __name__ == "system" and asserts it is blocked. This keeps the regression covered on the Linux CI runners where nt is unavailable. Fails on main, passes with this change.