fix(restore): allow restoring files from different source volume

The file restore command previously assumed that the target volume name
was identical to the volume name used in the backup path. This caused
restores to fail (exit code 2) when restoring data from one volume backup
into a different target volume.

Introduce an optional --source-volume argument for `baudolo-restore files`
to explicitly specify the backup source volume while keeping the target
volume unchanged.

- Default behavior remains fully backward-compatible
- Enables restoring backups from volume A into volume B
- Fixes E2E test scenario restoring into a new volume

Tests:
- Update E2E file restore test to use --source-volume

https://chatgpt.com/share/694ec70f-1d7c-800f-b221-9d22e4b0775e

tests/e2e/test_e2e_files_full.py

@@ -61,16 +61,26 @@ class TestE2EFilesFull(unittest.TestCase):
         cleanup_docker(containers=cls.containers, volumes=cls.volumes)
 
     def test_files_backup_exists(self) -> None:
-        p = backup_path(self.backups_dir, self.repo_name, self.version, self.volume_src) / "files" / "hello.txt"
+        p = (
+            backup_path(
+                self.backups_dir,
+                self.repo_name,
+                self.version,
+                self.volume_src,
+            )
+            / "files"
+            / "hello.txt"
+        )
         self.assertTrue(p.is_file(), f"Expected backed up file at: {p}")
 
     def test_restore_files_into_new_volume(self) -> None:
-        # restore files into dst volume
+        # restore files from volume_src backup into volume_dst
         run([
             "baudolo-restore", "files",
             self.volume_dst, self.hash, self.version,
             "--backups-dir", self.backups_dir,
             "--repo-name", self.repo_name,
+            "--source-volume", self.volume_src,
             "--rsync-image", "ghcr.io/kevinveenbirkenbach/alpine-rsync",
         ])