Kept missing-file and directory identity validation on Windows while skipping only the Unix mode-bit rejection.
Added regression tests for Windows missing and directory identity paths.
Fixes#2850
Skipped Unix mode-bit validation for ssh identity files on native Windows while preserving the stricter check for Unix-like platforms.
Added regression coverage for Windows ssh argument building with permissive low mode bits and for Unix rejection of group/world-readable keys.
Fixes#2850