fix: use shlex.join for safe command construction in snap-preseed plugin

[Copilot]

Command construction in get_build_commands mixed manual double-quote wrapping with string concatenation, combining the model assert path and install dir into a single list element — fragile for paths with spaces or shell-special characters.

Changes

• snap_preseed_plugin.py: Split model assert and install dir into separate cmd list items; replace " ".join(cmd) with shlex.join(cmd) for safe shell quoting
• test_snap_preseed_plugin.py: Update expected strings to match shlex.join output (empty model assert → ''; model.assert → unquoted)

Before:

cmd.append(f'"{options.snap_preseed_model_assert}" {self._part_info.part_install_dir}')
return [" ".join(cmd)]

After:

cmd.append(options.snap_preseed_model_assert)
cmd.append(str(self._part_info.part_install_dir))
return [shlex.join(cmd)]