Function process_file_arguments 

pub fn process_file_arguments(
    file_args: &[String],
    cwd: &Path,
    auto_resize_images: bool,
) -> Result<ProcessedFiles>

Process @file arguments into a single text prefix and image attachments.

Matches the legacy TypeScript behavior:

• Resolves paths (including ~ expansion + macOS screenshot variants)
• Skips empty files
• For images: attaches image blocks and appends <file name="...">...</file> references
• For text: embeds the file contents inside <file> tags