Improve SELinux Mode Detection with Enhanced GRUB Fallback Logic

[himaja-kesari]

This PR enhances the SELinux mode detection in BootCustomizer by implementing robust fallback logic between GRUB configuration variables and providing comprehensive error reporting. The changes improve reliability when detecting SELinux configuration in different GRUB setups, particularly for Fedora distributions.

Problem:
The existing SELinux mode detection logic in getSELinuxModeFromGrub() was fragile and could fail when:

GRUB_CMDLINE_LINUX variable was not present in grub
Only GRUB_CMDLINE_LINUX_DEFAULT contained the SELinux configuration

changes made :
Try to extract SELinux args from GRUB_CMDLINE_LINUX (higher priority) and also GRUB_CMDLINE_LINUX_DEFAULT
Enhanced error reporting: Capture and report both errors if both attempts fail

Checklist

• Tests added/updated
• Documentation updated (if needed)
• Code conforms to style guidelines