style: rustfmt the blob-removal code (fix the CI fmt gate)

The blob-removal commit (6b4c4dc) failed CI's fmt gate — long assert! lines
and a doc comment that stock rustfmt wraps. No behaviour change; just
`cargo fmt`. Verified via `nix develop -c cargo fmt --check`.

src/persistence/migrations.rs

@@ -394,8 +394,7 @@ mod tests {
             "      - \"note <> 'type: blob'\"\n",
             "relationships: []\n",
         );
-        let outcome =
-            migrate_to_latest(body, &MigratorRegistry::production(), tmp.path()).unwrap();
+        let outcome = migrate_to_latest(body, &MigratorRegistry::production(), tmp.path()).unwrap();
         assert_eq!(outcome.migrated_from, Some(1));
         assert!(outcome.body.contains("version: 2"));
         // The blob column became text.