Fix validators.disabled() to save/restore state on nesting

[bysiber]

Summary

validators.disabled() unconditionally re-enables validators in its finally block instead of restoring the previous state. This breaks nested usage — the inner context manager prematurely re-enables validators when it exits.

with disabled():
    with disabled():
        pass
    # BUG: validators are now ENABLED even though we're still in the outer disabled()
    C(x="not_int")  # raises TypeError unexpectedly

The fix saves the current state with get_run_validators() before disabling, then restores it in finally. This is a one-line semantic change.

The same pattern of nesting breakage also applies when set_run_validators(False) was called before entering disabled() — the context manager would unexpectedly override that.