Adopt IFileSystem abstraction across ImageBuilder

[lbussell]

PR #1957 introduces an IFileSystem abstraction for testability of the signing services. The rest of ImageBuilder still uses System.IO.File and System.IO.Directory directly.

Migrate all direct System.IO filesystem usage in src/ImageBuilder/ to use IFileSystem, extending the interface as needed. This enables unit testing without disk I/O and establishes a single consistent pattern.

Context: #1957 (comment)