Handle symlink entries correctly in ACR remote build context packing

[Copilot]

PackRemoteBuildSource treated all non-directory entries as regular files, so symlinked directories in the build context caused io.Copy to read a directory fd (EISDIR) and abort archive creation. It also generated incorrect symlink tar metadata by passing info.Name() instead of the symlink target to tar.FileInfoHeader.

• Symlink-aware tar header generation

  • Detect symlinks from FileMode.
  • Resolve link target via os.Readlink(path).
  • Pass the resolved target into tar.FileInfoHeader(info, linkTarget) so tar entries preserve correct Linkname.

• Payload copy restricted to regular files

  • Continue writing tar headers for included entries.
  • Only stream file bytes when info.Mode().IsRegular().
  • Avoid attempting to open/copy symlink entries (and other non-regular entries) as file payloads.

• Regression coverage for symlink contexts

  • Added focused tests for:
    • symlink → directory
    • symlink → file
  • Assertions validate tar entry type (TypeSymlink) and preserved link target.

linkTarget, err := getSymlinkTarget(path, info.Mode())
if err != nil {
	return err
}

hdr, err := tar.FileInfoHeader(info, linkTarget)
if err != nil {
	return err
}

if err := tw.WriteHeader(hdr); err != nil {
	return err
}

if info.Mode().IsRegular() {
	f, err := os.Open(path)
	if err != nil {
		return err
	}
	defer f.Close()

	if _, err := io.Copy(tw, f); err != nil {
		return err
	}
}